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| Illustration 1 | g03880704 |
| (A) Location of the service brake control valve | |
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| Illustration 2 | g03881293 |
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(1) Brake port for the rear service brakes
(3) Tank port for the rear axle (4) Brake port for the front service brakes (6) Tank port for the front axle (15) Accumulator port for the rear service brakes (20) Accumulator port for the front service brakes | |
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| Illustration 3 | g03880762 |
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Service brake control valve (1) Brake port for the rear service brakes (4) Brake port for the front service brakes (15) Accumulator port for the rear service brakes (20) Accumulator port for the front service brakes | |
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| Illustration 4 | g03880688 |
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View A-A (1) Brake port for the rear service brakes (3) Tank port for the rear axle (4) Brake port for the front service brakes (6) Tank port for the front axle (7) Modulating spring (8) Piston (9) Modulating spring (10) Brake pedal return spring (11) Upper spool (12) Retainer assembly (13) Pilot oil passage (14) Passage (15) Accumulator port for the rear axle (16) Cavity (17) Lower spool (18) Pilot oil passage (19) Cavity (20) Accumulator port for the front axle (21) Spring | |
The service brake control valve is a fully split modulating valve with two independent output pressures. The valve is actuated by a camshaft which can be actuated by either brake pedal.
The service brake control valve controls the modulation of high-pressure oil flow from the accumulators to the service brakes. The position of either brake pedal causes a specific pressure at the service brakes. As the position of the pedal changes, the pressure at the service brakes also changes.
When piston (8) is pushed downward by the linkage of the camshaft, pedal force acts to compress modulating spring (7), modulating spring (9) and brake pedal return spring (10). The compression of these springs forces spools (11) and (17) to move downward.
The flow of oil through passages (14) to the hydraulic tank is blocked when the spools move downward. When the spools move down farther, the accumulator oil pressure is applied to the service brakes. Passage (14) connects the accumulator oil pressure from accumulator ports (15) and (20) to the service brakes through brake ports (1) and (4).
As oil flows from the accumulators to the service brakes, oil also flows through passages (13) and (18) to cavities (16) and (19). The service brakes and the respective cavity have the same oil pressure.
The oil pressure in cavity (19) and the force of spring (21) act to balance lower valve spool (17) against the force created by the oil pressure in cavity (16). In the same manner, pressure in cavity (18) creates a force against the bottom of upper valve spool (11). This force is balanced by the force of modulation springs (7) and (9) at the top of the spool.
The force of springs (7), (9), and (10) act to balance the force that is applied to the brake pedal. Upward movement of valve spools (11) and (17) causes accumulator ports (15) and (20) to be covered. The level of pressure in the independent valve sections will create a force. This force is a type of feedback that allows the operator to modulate pressure to the service brakes.