Air System And Brakes

Figure 1 Air System And Brakes Schematic D25C 9YC284 - 9YC300 D30C 7ZC1 - 7ZC109 D350C 8XC131 - 8XC142

1. Parking Brake Actuator

2. Safety Valve

3. Air Dryer

4. System Protection Valve

5. Manifold

6. Air Filter

7. Parking/Secondary Brake Reservoir

8. Service Brake Reservoir

9. Service Brake Reservoir

10. Spool Valve

11. Relay Valve

12. Horn Valve

13. Horn

14. Differential Lock Valve

15. Parking Brake Valve

16. Pressure Gauges

17. Dual Brake Valve

18. Low Pressure Switches

19. Low Pressure Switch

20. Double Check Valve

21. Compressor

22. Relay Valve

23. Spring Brake Actuator

24. Quick Release Valve

25. Spool Valve

26. Spring Brake Actuator

27. Governor

28. Suspension Rotary Valve

29. Suspension Control Valve

30. Differential Lock Cylinder

31. Suspension Rotary Valve

32. Brake Fluid Reservoir

33. Air/Hydraulic Brake Actuator

34. Wheel Brake

35. Wheel Brake

36. Wheel Brake

37. Wheel Brake

38. Brake Fluid Reservoir

39. Air/Hydraulic Brake Actuator

40. Double Check Valve

41. Wheel Brake

42. Wheel Brake

43. Brake Fluid Reservoir

44. Air/Hydraulic Brake Actuator

The air and braking system piping arrangements are shown in figures 1 and 2.

An engine driven compressor (21) supplies air to dryer (3) which contains an internal non-return valve. The pressure in the air dryer is fed back to governor (27) which operates an unloader mechanism on the compressor to maintain the correct working pressure. A purge line "a" feeds unloader pressure back to the dryer again so that each time the compressor is unloaded the dryer is subjected to an internal reverse flow of air which regenerates the dryer element. The dryer has a safety valve (2) installed to protect against failure of the governor cut-out action.

When the pressure in the air dryer reaches a certain value, system protection valve (4) opens and air flows through the four outlets to the front and rear service brake reservoirs (8 and 9), parking/secondary brake reservoir (7) and manifold (5).

AIR SYSTEM COMPONENTS
3. Air Dryer 4. System Protection Valve 5. Manifold 6. Air Filter 7. Parking/Secondary Brake Reservoir 10. Spool Valve 25. Spool Valve 27. Governor.

Air flowing through manifold (5) flows to air filter (6) and horn valve (12). When the horn valve is operated air flows through the valve to sound the horn (13). Filter (6) is provided to clean the air being supplied to the machine suspension system.

Parking Brakes

The parking/secondary brake reservoir (7) is charged with air through one element of system protection valve (4). This air is used to supply relay valves (11 and 22) and parking brake valve (15).

When the parking brake valve is in the "OFF" position it directs pressurised air from line "b" to line "e". This air flows to hand control valve (14) and to:

1. the pilot port on relay valve (11). The relay valve opens and pressurised air flows through the valve to the front brake actuators (23 and 26) and to the parking brake actuator (1). Air directed to the front brake actuators flows through double check valve (20).

The power springs in the wheel brake actuators and parking brake actuator are compressed. This releases the front axle wheel brakes and the parking brake.

2. the pilot port on spool valve (25). The valve is piloted open and port 1 is connected to port 2 allowing air to pass to the front suspension rotary valve (28). This air supply ensures that the machine suspension system operates to provide automatic self levelling.

Air flowing to the spring chambers of the wheel brake actuators passes through quick release valve (24). A pressure switch (19) is screwed into the relay valve (11). The switch is a normally closed switch which is wired to a warning light in the cab. When air pressure is present (park brake "OFF") the switch is opened and the warning light is not illuminated.

When the parking brake valve is in the "PARK" position air in lines "c" and "e" is exhausted through line "d" and relay valves (11 and 22) are closed. Air pressure in the rear brake actuator(s) is exhausted at relay valve (22) and the rear wheel brakes are released. Air pressure in the spring chambers of the front axle brake actuators (23 and 26) is exhausted at quick release valve (24) and air in the parking brake actuator (1) is exhausted at relay valve (11).

The power springs in the brake actuators take over to apply the front wheel brakes and parking brake.

At the same time the return spring closes spool valve (25) to connect port 2 to port 3 and the air supply to the front suspension rotary valve (28) is blocked, thus isolating the rotary valve. This prevents the machine suspension from self-levelling when the parking brake is applied.

When the air pressure is released from the relay valve (11) low pressure switch (19) closes and the warning light in the cab illuminates to indicate that the parking brake is applied.

Figure 2 Air System And Brakes Schematic D25C 9YC301-Up D30C 7ZC110-Up D350C 8XC143-Up

1. Manifold 2. Suspension Control Valve 3. Pressure Switch 4. Pressure Switch

Inter-Axle Differential

When the parking brake is in the "OFF" position, hand control valve (14) receives pressurised air through line "e". When the valve is moved to the "disengaged" position air flows to actuating cylinder (30) which disengages the inter-axle differential lock clutch. When the valve is moved to the "engaged" position air in the actuating cylinder is exhausted and the differential lock clutch is engaged.

If the parking brake is applied, line "e" is open to exhaust so the inter-axle differential will be locked regardless of the position of the hand control valve.

Secondary Brakes

The parking/secondary brake reservoir (7) provides a supply of air for use in the secondary braking system.

When the parking brake valve lever is moved from the "OFF" position toward the "SEC" (secondary) position, air is directed from line "b" to line "c". The pressure in line "c" increases progressively as the valve lever is moved further away from the "OFF" position. Pressure in line "c" is directed to the pilot port of relay valve (22). The relay valve opens and allows air to flow through double check valve (40) to the rear air/hydraulic actuator(s).

The opening of relay valve (22) is proportional to the pressure in the pilot line "c". In turn the air pressure in the lines between the relay valve and the air/hydraulic actuator(s) is proportional to the opening of the relay valve. In this way a gradual application of the rear wheel brakes is provided as the parking brake valve lever is moved progressively toward the "SEC" (secondary) position.

At the same time the progressive increase in air pressure in line "c" is accompanied by a progressive reduction in the pressure in line "e". Pilot pressure to relay valve (11) reduces and the air pressure in the lines to the spring chambers of the front axle brake actuators and parking brake actuator decreases proportionally. Thus a gradual application of the front wheel brakes and parking brake occurs as the parking brake valve lever is moved progressively toward the secondary position. This results in a simultaneous application of the wheel brakes and parking brake.

Service Brakes

The service brake reservoirs (8 and 9) are charged with air from system protection valve (4). They supply air to the dual brake valve (17) which is operated by the service brake footpedal. When the footpedal is pressed air flows from the dual brake valve to:

1. the service chambers of the front axle spring brake actuators (23 and 26). The brake actuators then apply the front wheel brakes.

2. the rear air/hydraulic brake actuator(s) by way of double check valve (40). The rear wheel brakes are then applied.

3. the pilot port of spool valve (10). The valve is actuated and port 1 is connected to port 2. The air supply to the front suspension rotary valve is blocked by the valve spool. This prevents the suspension from self levelling when the service brakes are applied.

A stop light switch fitted to dual brake valve (17) closes to illuminate the stop lights during application of the service brakes. Low pressure switches (18) cause an alarm to sound when the air pressure in the front or rear systems falls below a certain value.

When the service brake is released the air supply is blocked at the dual brake valve (17). Air pressure in the service chambers of the front axle brake actuators and in the rear air/hydraulic brake actuator(s) is exhausted at the dual brake valve. Thus the wheel brakes are released.

When the air pressure in the line to the rear brake actuator(s) is exhausted, pilot pressure to spool valve (10) is removed. The return spring moves the valve spool to connect port 2 to port 3. Supply air passes to the front suspension rotary valve (28). The suspension will again provide automatic self levelling.

On later machines (figure 2) the suspension levelling system is controlled electrically. The air filter, spool valves and rotary valve are not fitted.

An air supply is taken directly from the manifold (1) on the system protection valve to the suspension control valve (2). Pressure switch (3) prevents the suspension from levelling when the secondary brakes are applied, and pressure switch (4) prevents the suspension from levelling when the service brakes are applied.

NOTE: For detailed information on the operation of the machine suspension air systems refer to the appropriate service manual module.

Air Compressor

AIR COMPRESSOR
1. Crankshaft 2. Piston Head 3. Inlet Valve 4. Spring 5. Discharge Line 6. Spring 7. Discharge Valve 8. Plunger 9. Saddle Spring 10. Unloader Piston.

The two cylinder, single stage, reciprocating compressor is driven through a power take off on the engine.

During the piston down stroke a vacuum is created above the piston head (2) which causes the inlet valve (3) to lift off its seat. This allows air to flow from the engine inlet manifold, past the inlet valve to the compressor cylinder. When the piston reaches the end of its down stroke spring (4) forces the inlet valve back against its seat.

During the piston up stroke the air drawn into the cylinder is being compressed. Discharge valve (7) is forced off its seat and compressed air flows into the air dryer. When the piston reaches the end of its up stroke the pressure in the discharge line (5) together with the effect of spring (6) forces discharge valve (7) against its seat.

When the pressure in the air dryer reaches the governor cut-out setting, the governor allows air to flow from the dryer to a cavity below the unloader pistons (10). This lifts the unloader pistons and plungers (8). The plungers hold the inlet valves off their seats so that air flows between the two cylinders. The compressor is then off load.

When the air dryer pressure falls to the governor cut-in setting, the air holding the unloader plungers is exhausted. The unloader saddle spring (9) forces the plungers and pistons down, and the inlet valves can then return to their seats. The compressor is then on load again.

Governor

GOVERNOR
1. Conical Spring 2. Piston 3. Disc Valve 4. Chamber 5. Wire Mesh Filter 6. Inlet Port 7. Passage 8. Unloader Port 9. Wire Mesh Filter 10. Passage 11. Exhaust Stem 12. Purge Port 13. Exhaust Port 14. Control Spring

Air from the dryer enters the governor through inlet port (6) and flows through wire mesh filter (5) to chamber (4). Pressure in chamber (4) acts against the underside of piston (2). As the pressure in the air dryer rises the piston rises until disc valve (3) comes into contact with the spring loaded exhaust stem (11). A further increase in pressure results in further upward movement of the piston and the exhaust stem pushes disc valve (3) off its seat. Air then flows from chamber (4) past the disc valve and into passage (7). Passage (7) is connected to passage (10) at this point so that air flows through wire mesh filter (9) to the compressor unloader port (8) and the air dryer purge port (12). The unloader port directs air to the compressor unloader mechanism and the compressor cuts out. The purge port directs air back to the dryer to initiate the purge cycle.

As the dryer pressure falls the piston (2) moves downward under the effect of control spring (14) allowing disc valve (3) to return to its seat. This isolates chamber (4) from passage (10). As the exhaust stem (11) lifts clear of the disc valve air pressure in passage (10) is exhausted through the hollow exhaust stem and port (13). The compressor then cuts in to recharge the air dryer.

Air Dryer

AIR DRYER
1. Check Valve 2. Outlet Port 3. Purge Chamber 4. Check Valve 5. Safety Valve Port 6. Filter 7. Heater 8. Silencer 9. Purge Valve 10. Spring 11. Purge Line Port 12. Piston 13. Dryer Body 14. Desiccant Cartridge 15. Inlet Port 16. Purge Vent

The air dryer cools, filters and dries the pressurised air delivered by the compressor.

Compressed air entering the dryer will always have a very high water vapour content. Air enters through inlet port (15) and flows downward between the dryer body (13) and cartridge (14). As this air is cooled condensate forms and collects in the bottom of the dryer. Most of the dirt in the air settles out at this stage. The air then passes up through the filter (6) which removes most of the oil and water droplets remaining in suspension in the air. Any remaining particles of dirt are also trapped by the filter.

Cartridge (14) is filled with thousands of tiny pellets. As the air passes through the cartridge, water vapour is removed by a process known as adsorbtion. This moisture collects in the pores of the micro-crystalline structure of the pellets (called desiccant pellets).

After passing through the desiccant the dry air passes through check valve (4) and enters the purge chamber (3). From the purge chamber air passes through check valve (1) and outlet port (2) to the system protection valve and reservoirs.

When the pressure in the reservoirs and purge chamber is high enough the governor operates to unload the compressor. At the same time the governor feeds pressure to port (11). This pressure forces piston (12) downward against the effect of spring (10). The opening of the valve allows the existing pressure in the dryer to force accumulated condensate and dirt out through the vent. The air in the purge chamber expands back through purge vent (16) and the desiccant cartridge to dry it out. In escaping past the open purge valve (9) the air carries this moisture with it. The desiccant material is regenerated in this way each time the compressor is taken off load, thus preventing it from becoming totally saturated with moisture. Check valve (1) prevents reservoir pressure from blowing back through the dryer.

Silencer (8) reduces noise during the purge cycle and a thermostatically controlled heater (7) prevents condensate from freezing.

Safety Valve

SAFETY VALVE
1. Spring 2. Ball 3. Seat 4. Exhaust Port

Air pressure in the dryer acts against the underside of ball (2). If the governor cut-out action fails the pressure will rise until the effect of spring (1) is overcome and ball (2) lifts off its seat (3) to exhaust air through port (4).

System Protection Valve

SYSTEM PROTECTION VALVE
1. Chamber 2. Passage 3. Outlet Port 4. Check Valve 5. Spring 6. Outlet Port 7. Check Valve (Seat) 8. Chamber 9. Valve Element (Auxillaries Manifold) 10. Outlet Port 11. Valve Element (Parking/Secondary Reservoir) 12. Chamber 13. Outlet Port 14. Chamber 15. Inlet Port 16. Valve Element (Service Brakes) 17. Valve Element (Service Brakes)

Air from the dryer enters the valve through port (15) and passes to chambers (14) and (1) in the valve body. This pressure acts against pistons (30) which rise against the effect of springs (25). Air then flows past the valve seats (19) to the front and rear service brake reservoirs through ports (3) and (13). Air also flows past check valves (4) and (7) to chamber (8).

Once the opening pressure of pistons (30) has been reached and they lift off their seats, inlet pressure acts against the full face of the pistons, keeping them raised against their springs. Normally, the pistons will remain raised and the service brake reservoirs will be interconnected. This permits the equalisation of air pressure in the service brake reservoirs and the air dryer and enables the governor to control service brake pressure directly.

SYSTEM PROTECTION VALVE
18. Spring Retainer 19. Valve Seat 20. Valve Seat 21. Piston Insert 22. Hole 23. Breather Cap 24. Adjusting Screw 25. Spring 26. Spring 27. Piston 28. Check Valve 29. Piston Insert 30. Piston

Air in chamber (8) is fed to chambers (12) - one for each valve (9) and (11) - by internal passages. This air acts against check valves (28) which lift off their seats (20). Once the check valves lift, air pressure acts against the faces of pistons (27), which lift against springs (25). Air then flows through outlet port (10) to the parking brake reservoir and through port (6) to the manifold which supplies the horn valve, differential lock valve, tire inflator connector (if fitted) and suspension filter. The pistons (27) are kept raised against their springs by inlet pressure. When the inlet air supply is interrupted, such as when the governor cuts out the compressor, check valves (28) close against their seats (20) to isolate ports (6) and (10) from each other and from the other two ports (3) and (13). Check valves (28) then have inlet pressure acting against their front faces and outlet pressure acting against their back faces as well as springs (26). If the outlet pressure falls the check valves will lift to recharge the system.

If there is a failure in one of the service brake circuits the reservoir pressure will fall and pistons (30) will fall until they are fully closed. The pressure at which they are fully closed is lower than their initial opening pressure because inlet air acts against the full face area when the pistons are open and against reduced area when they are closed. Once fully closed the inlet pressure will rise again to the value required to initially lift pistons (30) from their seats. The unaffected service brake circuit is thus assured of a continued supply of air at not less than this initial opening pressure. The check valve (4) or (7) between the affected valve piston and chamber (8) will also close, preventing loss of air from the chamber.

If there is a failure in one of the circuits fed from port (6) or (10) the piston (27) will fall and close off that circuit. The remaining circuits will continue to receive air at the initial opening pressure of the pistons.

The initial opening pressure of the pistons is controlled by adjusting screws (24) which determine the pre-load in springs (25). The spring chambers are vented to atmosphere through holes (22) and breather caps (23).

Relay Valve

RELAY VALVE
1. Cover 2. Pilot Port 3. Piston 4. Body 5. Exhaust Seat 6. Pressure Passage 7. Spring 8. Inlet Seat 9. Outlet Chamber 10. Spring

Pilot pressure enters the cover (1) through port (2). System pressure from the parking brake reservoir enters the body (4) through passage (6). Outlet chamber (9) passes pressurised air to the parking brake actuator and to the wheel brake actuators.

Pilot pressure acting against piston (3) moves the piston downward until exhaust seat (5) is covered. Further downward movement compresses spring (7) and uncovers the inlet seat (8). Air from the reservoir can then flow into outlet chamber (9). The pressure in outlet chamber (9) is felt against the under-side of piston (3). This, combined with the effect of spring (10) moves the piston upward until the relay valve is in a state of balance. Exhaust seat (5) remains covered and the effect of spring (7) causes inlet seat (8) to be covered. If the pilot pressure is increased (due to further movement of the parking brake valve) the relay valve will re-open and pass more air to outlet chamber (9). As the pressure in the outlet chamber increases the relay valve will move back into a state of balance.

If pilot pressure in port (2) is reduced, spring (10) moves piston (3) upward, opening exhaust seat (5). Air pressure in outlet chamber (9) is reduced as air flows to exhaust until spring (7) moves inlet seat (8) back into contact with exhaust seat (5). The valve is then back in a state of balance with reduced pressure in outlet chamber (9).

The balancing action of the relay valve results in the progressive application and release of the parking and/or secondary braking systems.

Dual Brake Valve

When the service brake footpedal is pressed force is transmitted through spindle (1), cam (24) and plunger (2) to spring seat (22) and rubber spring (23). This force moves piston (20) downward and exhaust seat (19) closes against valve (17). Further downward movement pushes valve (17) away from inlet seat (18) allowing air to flow from inlet port (4) to outlet port (16).

Air passing through outlet port (16) also flows through transfer holes (15) and acts against the relay piston (12). The downward movement of piston (20) and valve (17) results in an additional force being applied to piston (12) by spring (14). Piston (12) moves downward and exhaust valve seat (11) closes against valve (8). Valve (8) moves away from inlet seat (10) allowing air to flow from inlet port (5) to outlet port (9).

If the brake footpedal is held in one position the air pressure under piston (20) will increase until it combines with the upward forces exerted by springs (21) and (3) to balance the force exerted on the footpedal. The piston (20) and valve (17) then lift sufficiently to allow inlet seat (18) to contact the valve. At this point exhaust seat (19) and inlet seat (18) are both covered and the valve element (17) is in a state of balance. Similarly, increasing pressure under piston (12) combined with the effect of spring (7) overcomes the downward force exerted by spring (14) and air pressure acting above piston (12). The piston then lifts sufficiently to allow valve (8) to close against inlet seat (10) while keeping exhaust seat (11) covered. The lower valve element (8) is then also in a state of balance.

DUAL BRAKE VALVE
1. Spindle 2. Plunger 3. Spring 4. Inlet Port 5. Inlet Port 6. Rubber Flap 7. Spring 8. Valve Element 9. Outlet Port 10. Inlet Seat 11. Exhaust Seat 12. Piston 13. Passage 14. Spring 15. Hole 16. Outlet Port 17. Valve Element 18. Inlet Seat 19. Exhaust Seat 20. Piston 21. Spring 22. Spring Seat 23. Rubber Spring 24. Cam 25. Footpedal

If the brake application is increased the downward force exerted on piston (20) increases. Valve (17) is pushed off inlet seat (18) allowing more air to flow from inlet port (4) to outlet port (16). The outlet pressure then increases to give increased braking effect. Valve (8) also moves away from inlet seat (10) in the manner described previously. Thus the pressure in outlet port (9) also increases until both elements again move into a state of balance.

When the brake application is reduced the downward force exerted on piston (20) is reduced and the piston moves upward under the influence of pressure in port (16) and spring (21). Exhaust seat (19) moves away from valve (17) allowing air to exhaust through passage (13) and past the rubber flap (6). This action continues until the pressure in port (16) falls sufficiently to restore the balance of forces on piston (20). As the pressure in port (16) falls the force exerted on the upper face of piston (12) also falls allowing the piston to rise. Exhaust seat (11) moves away from valve (8) allowing air to exhaust from port (9) through passage (13). The pressure in port (9) then falls until the forces on piston (12) balance again.

The self balancing action of the valve controls the outlet pressures in ports (9) and (16) at levels proportional to the force exerted on the footpedal.

Spring Brake Actuator - Front Brakes

SPRING BRAKE ACTUATOR
1. Spring Housing 2. Snap Ring 3. Power Spring 4. Piston 5. Spring Brake Chamber 6. Service Chamber 7. Clamp Ring 8. Diaphragm 9. Spring 10. Pushrod 11. Mounting Studs 12. Breather Tube 13. Seals 14. Piston Rod 15. Cap 16. Boss 17. Release Bolt 18. Anti-explosion Washer

When the parking brake valve is in the "OFF" position and the service brake footpedal is released air enters the spring brake chamber (5). Air pressure acting against the piston (4) moves it to the left against the effect of the power spring (3). At the same time spring (9) moves pushrod (10) to the left, thus releasing the brakes.

When the service brake footpedal is applied air flows from the dual brake valve into the service chamber (6). Air pressure acting against diaphragm (8) moves pushrod (10) to the right against the effect of spring (9). Thus the brakes are applied. As long as the parking brake valve remains in the "OFF" position air pressure in the spring brake chamber (5) will hold piston (4) left. As the parking brake valve is moved toward the "SECONDARY" or "PARK" position air pressure in the spring brake chamber will be progressively reduced due to air being exhausted at the quick release valve. The power spring (3) moves piston (4) to the right by an amount depending on the pressure remaining in the spring brake chamber, thus providing a gradual application of the brake.

When the parking brake valve is in the the parking brake valve is in the "SECONDARY" or "PARK" position all air pressure is exhausted from the spring brake chamber and the brake is fully applied.

If there is a failure of the air supply the power spring (3) will apply the brake. The brake can be manually released by removing cap (15) and turning the release bolt (17). As the bolt unscrews, spring (9) forces pushrod (10) and piston rod (14) to the left, releasing the brake.

Air/Hydraulic Brake Actuator - Rear Brakes

AIR/HYDRAULIC BRAKE ACTUATOR
1. Bleed Screw 2. Master Cylinder 3. Passage 4. Valve 5. Reservoir Port 6. Microswitch 7. Overstroke Plunger 8. Piston 9. Air Inlet Port 10. Rod 11. Spring 12. Valve Insert 13. Power Piston 14. Brake Fluid Port.

Compressed air from the dual brake valve enters the actuator through port (9) and acts against piston (8). Piston (8) and rod (10) move to the left against the effect of spring (11). Rod (10) pushes against valve (4) and valve insert (12), which closes passage (3). Further movement of the piston and rod assembly forces power piston (13) to the left and displaces brake fluid from master cylinder (2). This fluid flow through outlet port (14) to the slave cylinders which apply the brakes.

When the dual brake valve footpedal is released air pressure is exhausted from port (9). Piston (8), rod (10) and power piston (13) move to the right under the influence of high pressure brake fluid in the master cylinder and return spring (11). The brake fluid reservoir is connected to port (5) which allows "make-up" fluid to flow across the flats on valve (4), past the valve insert (12) and through passage (3) to master cylinder (2).

If there is a leak of hydraulic fluid the movement of power piston (13) to the left will fail to generate the hydraulic fluid pressure necessary to balance the air pressure behind piston (9). Therefore, the piston will continue to move to the left and will eventually push overstroke plunger (7) to the left. This plunger will close microswitch (6) which completes an electrical circuit. A flashing light on the main dashpanel then warns the operator.

A bleed screw (1) is provided to purge the master cylinder of air.

Front Wheel Brakes

The front wheel brakes are air operated drum type brakes. The brake drum (5) is fastened to the wheel assembly. When the brake pedal is pressed air under pressure is directed to the spring brake actuators. The actuator push rods operate the brake cylinders (9) which in turn operate the brake shoes (6) which pivot about pins (4) and are forced into contact with the drum.

The brake actuator push rod (11) acts against follower (21) and piston (12). As the piston moves downward it pushes rollers (15) downward which bear against the tapered edges of plungers (19). The plungers then force the adjusting screws (8) out of the cylinder which force the brake shoe assemblies (6) into contact with the drum.

BRAKE ASSEMBLY
1. Cylinder 2. Spring 3. Spider 4. Pivot Pin 5. Brake Drum 6. Brake Shoe Assembly

BRAKE ASSEMBLY
2. Spring 3. Spider 4. Pivot Pin 6. Brake Shoe Assembly 7. Brake Lining 8. Adjusting Screws 9. Brake Cylinder

BRAKE CYLINDER ASSEMBLY
1. Cylinder 8. Adjusting Screw 10. Brake Actuator 11. Pushrod 12. Piston 13. Seal 14. Plug 15. Roller 16. Spring 17. Pawl 18. Actuator 19. Plunger 20. Spring 21. Follower.

When the brake is released spring (20) forces piston (12) and rollers (15) upward again. At the same time spring (2) pulls the brake shoes away from the drum and forces the adjusting screws (8) and plungers (19) back into the cylinders.

Adjusting screws (8) are threaded into actuators (18) which are a free fit in plungers (19). Plugs (14), springs (16) and pawls (17) act as guide assemblies for the plungers (19). The ends of the pawls are toothed and engaged with helical teeth on the outside diameter of actuators (18). As the actuators move out of the cylinder during a brake application the helical teeth on the actuators lift the pawls (17) against the effect of springs (16). When the brake is released the actuators (18) and pawls (17) return to their original position.

As the brake linings (7) wear the stroke of the actuators (18) and hence the lift of the pawls (17) gradually increases. Eventually the pawls (17) climb over the tooth in the actuators and drop into the next tooth space. When the brakes are subsequently released and the plungers move back into the cylinder, the teeth on the pawl cause the actuator to turn and move outward. Thus the adjusting screw moves outward and reduces the clearance between the brake lining and drum.

Rear Wheel Brakes

The rear wheel brakes are hydraulically operated drum type brakes. The wheel brake assembly is the same as that shown in 'Front Wheel Brakes' except that cylinder (1) is a slave cylinder. It is actuated by pressurised brake fluid from the master cylinder of the air/hydraulic brake actuator.

Brake hydraulic fluid enters the slave cylinder through inlet port (22) and acts against piston (12). As the piston is forced downward it pushes rollers (15) downward which bear against the tapered edges of plungers (19). The plungers then force the adjusting screws (8) out of the cylinder. This in turn forces the brake shoe assemblies into contact with the drum.

When the brake is released spring (20) forces piston (12) and rollers (15) upward again and brake fluid is forced through port (22) to the brake actuator master cylinder. At the same time spring (2) pulls the brake shoes away from the drum and forces the adjusting screws (8) and plungers (19) back into the cylinder.

SLAVE CYLINDER ASSEMBLY
1. Cylinder 8. Adjusting Screw 12. Piston 13. Seal 14. Plug 15. Rollers 16. Spring 17. Pawl 18. Actuator 19. Plunger 20. Spring 22. Inlet Port 23. Bleed Screw.

Adjusting screws (8) are threaded into actuators (18) which are a free fit in plungers (19). Plugs (14), springs (16) and pawls (17) act as guide assemblies for the plungers (19). The ends of the pawls are toothed and engaged with helical teeth on the outside diameter of actuators (18). As the actuators move out of the slave cylinder during a brake application the helical teeth on the actuators lift the pawls (17) against the effect of springs (16). When the brake is released the actuators (18) and pawls (17) return to their original position.

As the brake linings (7) wear the stroke of the actuators (18) and hence the lift of the pawls (17) gradually increases. Eventually the pawls (17) climb over the tooth in the actuators and drop into the next tooth space. When the brakes are subsequently released and the plungers move back into the cylinder, the teeth on the pawl cause the actuator to turn and move outward. Thus the adjusting screw moves outward and reduces the clearance between the brake lining and drum.

Quick Release Valve

QUICK RELEASE VALVE
1. Inlet Port 2. Inlet Seat 3. Outlet Port 4. Exhaust Seat 5. Exhaust Port 6. Diaphragm 7. Outlet Port

Air enters the valve through inlet port (1) and forces diaphragm (6) downward, opening inlet seat (2) and closing exhaust seat (4). The air then flows through outlet ports (3) and (7) to the brake chambers.

When inlet pressure is removed from port (1) - due to the service brake pedal being released or the park brake valve being moved to the "PARK" position - pressure in the outlet ports (3) and (7) moves the diaphragm up to open exhaust seat (4) and close inlet seat (2). Air in the actuators is exhausted through port (5).

Horn Control Valve

HORN CONTROL VALVE
1. Button 2. Plunger 3. Inlet Port 4. Seat 5. Valve 6. Ball 7. Spring 8. Strainer 9. Outlet Port.

Air enters the valve through inlet port (3). When the button (1) is depressed plunger (2) unseats valve (5) from seat (4) and air flows through outlet port (9) to the horn. When the button is released the spring (7) pushes valve (5) back into contact with seat (4).

Low Pressure Switch

LOW PRESSURE SWITCH
1. Contact Disc 2. Piston 3. Passage 4. Diaphragm 5. Fixed Contact 6. Control Spring.

Air enters the switch through inlet passage (3) and acts against diaphragm (4). The piston (2) and contact disc (1) move upward against the effect of the control spring (6), thus breaking the electrical circuit. This is the normal running condition.

If the supply pressure falls below the setting pressure the control spring moves the contact disc down against the fixed contact (5) to complete the circuit. The switch is fitted to give warning of low air pressure in the service and parking brake circuits.

Stop Light Switch

STOP LIGHT SWITCH
1. Spring 2. Piston 3. Diaphragm/Seal 4. Inlet Port 5. Contact Strip 6. Terminal.

Whenever the service brake valve is operated air pressure is applied to inlet port (4). This acts against the combined diaphragm/seal (3) which moves piston (2) upward against the effect of spring (1). Contact strip (5) then bridges the gap between the terminals (6) to complete an electrical circuit and illuminate the stop lights.

The switch contacts close at approximately 35 kPa (5 p.s.i.), thus ensuring that the stop lights illuminate immediately a brake application is made.

Differential Lock Valve

DIFFERENTIAL LOCK VALVE
1. Lever 2. Cam 3. Spring 4. Valve 5. Spring 6. Inlet Port 7. Exhaust Seat 8. Inlet Seat 9. Outlet Port 10. Plunger 11. Exhaust Outlet

When the valve is in the "disengaged" position the cam (2) on the bottom of the lever (1) depresses plunger (10). The plunger moves downward closing exhaust seat (7) against valve (4) and lifting the valve from the inlet seat (8). Air then passes through inlet port (6) to outlet port (9) and on to the differential lock cylinder.

When the lever (1) is moved to the "engaged" position, spring (3) moves plunger (10) upward. This opens exhaust seat (7) and allows spring (5) to move valve (4) upward against the inlet seat, thus closing off the air supply. Air already in the differential lock cylinder flows past exhaust seat (7) to atmosphere through outlet (11).

Differential Lock Cylinder

DIFFERENTIAL LOCK CYLINDER
1. Rod 2. Inlet Port 3. Piston 4. Breather Port 5. Spring.

When the "disengaged" position is selected air enters the cylinder through inlet port (2). Piston (3) and rod (1) move to the right against the effect of spring (5). The rod is attached to the dog clutch selector shaft. The dog clutch is then disengaged and the inter axle differential operates normally.

When the "engaged" position is selected port (2) is connected to exhaust. Spring (5) then pushes rod (1) to the left. The dog clutch is then engaged and the inter axle differential is locked.

Parking Brake Valve

Air from the parking/secondary brake reservoir enters the valve through an inlet port and flows through holes (7 and 18).

When the operating handle (1) is in the "OFF" position cam arrangement (3) holds piston (5) down and allows piston (21) to rise fully. The position of piston (5) is such that exhaust seat (8) is covered and plunger (9) is moved downward, uncovering inlet seat (6). Thus air flows past inlet seat (6) to an outlet port which leads to the pilot port of the relay valve controlling the parking brakes. The relay valve opens and pressurised air flows to the parking brake actuator and to the spring chambers of the front axle wheel brake actuators. The power springs in the actuators are compressed and the parking brakes are released. The position of piston (21) is such that exhaust seat (20) is open and inlet seat (19) is covered. The port leading to the pilot port of the relay valve controlling the secondary operation of the rear wheel brakes is connected to exhaust passage (13) through the hollow plunger (17). Therefore the relay valve is closed, preventing application of the rear wheel brakes through the secondary braking system.

As the handle is moved from the "OFF" position toward the "SEC" (secondary) position it rotates the cam arrangement which simultaneously allows piston (5) to rise and depresses piston (21). As piston (21) is depressed exhaust seat (20) is covered and inlet seat (19) is uncovered. This allows air to flow to the pilot port of the relay valve controlling the application of the rear wheel brakes through the secondary braking system. The pressure to the relay valve pilot port also acts against the top of piston (16) and causes it to move downward against the force exerted by spring (15). As the piston moves downward inlet seat (19) is covered and the pressure acting on the top of piston (16) is balanced by the force of the spring (15). In this position both the inlet and exhaust seats are covered and the valve assembly is in a state of balance. The pilot pressure to the relay valve causes it to open by an amount proportional to the pilot pressure. The relay valve then permits air from the parking/secondary brake reservoir to flow to the rear wheel brake actuators at a pressure proportional to the opening of the relay valve.

PARKING BRAKE VALVE
1. Handle 2. Locking Collar 3. Cam 4. Cam Follower 5. Piston 6. Inlet Seat 7. Holes 8. Exhaust Seat 9. Plunger 10. Piston 11. Spring 12. Holes 13. Exhaust Passage 14. Holes 15. Spring 16. Piston 17. Plunger 18. Holes 19. Inlet Seat 20. Exhaust Seat 21. Piston 22. Cam Follower.

At the same time piston (5) rises uncovering exhaust seat (8) and allowing air pressure in the pilot line to the relay valve (controlling the parking brakes) to flow to exhaust passage (13) through the hollow plunger (9). As the pressure falls, spring (11) moves piston (10) upward until exhaust seat (8) is covered again. In this position both the inlet and exhaust seats are covered and the valve assembly is in a state of balance. As the pressure in the pilot line falls the relay valve begins to close. The power springs in the parking brake actuator and front axle wheel brake actuators start to apply the parking brakes.

This action continues as the handle is moved toward the "SEC" (secondary) position so that a progressive pressure increase in the line to the rear air/hydraulic brake actuators is accompanied by a progressive pressure reduction in the line to the parking brake actuator and spring chambers of the front wheel brake actuators. Therefore a progressive application of the rear wheel brakes occurs at the same time as the front wheel brakes and parking brake are progressively applied.

When the handle is in the "SEC" position piston (5) is fully up and piston (21) is fully down. Inlet seat (6) is covered and exhaust seat (8) is uncovered. The pilot line to the relay valve controlling the parking brakes is open to exhaust and the power springs take over to fully apply the parking brakes. Inlet seat (19) is uncovered and exhaust seat (20) is covered so that full system pressure is delivered to the relay valve controlling the secondary operation of the rear wheel brakes. The relay valve opens fully and passes full system pressure to the air/hydraulic actuators to fully apply the rear wheel brakes.

When the handle is moved from the "SEC" position to the "PARK" position the cam shape is such that piston (21) rises until exhaust seat (20) is uncovered and the pilot pressure to the relay valve controlling the secondary operation of the rear wheel brakes is exhausted. The relay valve then closes, releasing pressure from the air/hydraulic actuators and the rear wheel brakes are released. Piston (5) remains in the raised position so that the parking brakes remain fully applied.

Double Check Valve

DOUBLE CHECK VALVE
1. Inlet Port 2. Shuttle Valve 3. Inlet Port 4. Outlet Port.

Air enters the double check valve through either of the inlet ports (1 or 3). Shuttle valve (2) moves either to the left or right and allows air to flow through outlet port (4). If air is available at both inlet ports simultaneously the shuttle valve (2) will respond to and deliver air from the port containing the higher pressure.

Parking Brake Actuator

PARKING BRAKE ACTUATOR
1. Diaphragm 2. Spring Chamber 3. Yoke 4. Push Rod 5. Power Spring 6. Push Plate 7. Breather Tube 8. Air Inlet Port

When the park brake valve is in the "OFF" position air enters the actuator through port (8). Air pressure acts against diaphragm (1) and moves push plate (6) and rod (4) outward. Yoke (3) is attached to the park brake assembly which is released.

When the park brake valve is in the "PARK" (or "SECONDARY") position air pressure is exhausted and power spring (5) moves the push plate and rod inward. The brake is then applied.

Parking Brake Assembly

PARKING BRAKE ASSEMBLY (1)

PARKING BRAKE ASSEMBLY (2)
1. Brake Actuator 2. Yoke 3. Links 4. Pin 5. Balls 6. Rotating Discs 7. Transfer Drive Top Shaft 8. Springs 9. Fixed Plates 10. Pin 11. Oil Seal 12. Oil Seal.

The park brake actuator (1) is fixed to the brake links (3) by pin (4) and yoke (2). Rotating discs (6) are spline connected to the output transfer drive top shaft (7) and the fixed plates (9) are held stationary by pin (10).

When the park brake is released the actuator rod is fully extended and three springs (8) pull the two plates (9a) and (9b) together. The three discs are then free to rotate between plates (9). When the park brake is applied the actuator rod moves upward. Links (3) also move upward causing the two plates (9a) and (9b) to pivot inward. Balls (5) run in tapered grooves so that the plates are forced apart. The discs (6) are then clamped between the plates preventing the output transfer drive top shaft from rotating.