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| Illustration 1 | g00910047 |
Service Brake Control Valve | |
Gradual modulation of the service brake system is controlled by the use of the foot operated control valve. The rate of rise or modulation is equal to time versus psi.
The foot operated valve modulates air pressure from the service air reservoirs to the brake relay valves. The valve body serves as a junction point for the air circuits with nonmodulated components and with the retarder brake control valve.
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| Illustration 2 | g00910090 |
Service Brake Control Valve (1) Seat (2) Spring (3) Piston assembly (4) Outlet passage (5) Spring (6) Valve seat (7) Valve seat (8) Valve (9) Valve body (10) Valve spring (11) Exhaust passage (12) Inlet passage (13) Exhaust diaphragm | |
When the brake pedal is pushed, a force is placed on seat (1). This force pushes down rubber spring (2) and piston assembly (3). Valve seat (6) closes exhaust passage (11) in valve (8). Piston assembly (3) moves valve (8) off valve seat (7). Pressure air from inlet passage (12) flows around valve (8) to outlet passage (4). The air then flows to the relay valve for the air chambers in the brake master cylinders.
When the air pressure below piston assembly (3) is larger than the force above the piston, the piston will rise in order to allow valve (8) to make contact with valve seat (7). This stops the supply of pressure air. Piston assembly (3) is still in contact with valve (8), so exhaust passage (11) is also closed. The service brake control valve is now in balance. Air pressure is held in the lines and in the air chambers in the brake master cylinders.
When the pedal is lifted, the force above piston assembly (3) decreases. The pressure air below piston assembly (3) and the force of spring (5) lifts the piston off valve (8). Pressure air in the lines and in the air chambers of the brake master cylinders flows around piston assembly (3) ., The pressure then flows through exhaust passage (11) and out exhaust diaphragm (13). The pressure air will flow until the forces above the piston and below the piston are balanced.
When the pedal is completely released, piston assembly (3) moves off valve (8). Air pressure opens exhaust diaphragm (13). Valve (8) is held against valve seat (7) by spring (10).