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| Illustration 1 | g00984823 |
The service brake control valve is located under the service brake pedal in the front of the cab.
The service brake control valve is actuated by depressing the service brake pedal in the cab. When the brake pedal is depressed, a roller on the pedal assembly actuates the service brake control valve.
The service brake control valve is a dual pressure reducing valve with two independent output pressures.
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| Illustration 2 | g00880683 |
Service brake control valve (B) Service brake ports (C) Pressure ports (from accumulators) | |
Modulation of the pressurized oil in the accumulators to the service brakes is controlled by the service brake control valve. The position of the brake pedal causes a specific pressure at the service brakes. As the position of the pedal changes, the pressure at the service brakes also change.
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| Illustration 3 | g00880685 |
View (A-A) of the service brake control valve (1) Piston (2) Spring (3) Spring (4) Spring (5) Retainer (6) Ball (7) Spacer (8) Upper valve spool (9) Plug (10) Outlet to the rear brakes (11) Passage (12) Drain port to tank (13) Supply from accumulator for the rear service brakes (14) Upper valve spool passage (15) Cavity (16) Lower valve spool (17) Outlet to tank (18) Outlet to front service brakes (19) Passage (20) Lower valve spool passage (21) Supply from accumulator for the front service brakes (22) Spring (23) Cavity | |
When the operator pushes on the brake pedal, a roller on the brake pedal assembly forces piston (1) downward. Piston (1) applies force on springs (2), (3) and (4). The force of springs (2) and (3) moves retainer (5) and ball (6). Movement of retainer (5) and ball (6) causes upper spool (8) to move away from the seat in spacer (7) .
The movement of upper valve spool (8) causes the movement of the lower valve spool (16) and the compression of spring (22). When valve spools (8) and (16) move, oil flow through passages (11) and (19) to the hydraulic tank is blocked.
This allows pressurized oil from accumulator ports (13) and (21) to flow through passage's (11), (14), (19) and (20). The oil then flows through passages (10) and (18) in order to apply the service brakes. At the same time, pressurized oil flows into cavities (15) and (23) and through passages (14) and (20). The oil pressure at the service brakes has the same pressure as the oil in cavities (15) and (23) .
Oil pressure in cavity (23) and the force of spring (22) act to balance lower valve spool (16) against the force of the pressure in cavity (15). In the same manner, pressure in cavity (15) creates a force against the bottom of upper valve spool (8). The upper valve spool is balanced by the force of springs (2) and (3) at the top of spool (8) .
The force of springs (2), (3) and (4) is balanced by the force that is applied to the pedal. Upward movement of valve spools (8) and (16) and causes accumulator ports (13) and (21) to be covered. The level of pressure in the independent valve sections create a feedback force. This allows the operator to modulate the pressure to the service brakes.
Valve spools (8) and (16) balance between the ports for the hydraulic tank and the ports for the service brakes. This is done in order to maintain the brake pressure until the position of the pedal changes. The change in the pedal position means that more brake pressure or less brake pressure is necessary.
If piston (1) is moved downward in order to apply more compression on springs (2) and (3), valve spool (8) and (16) will move. The valve spools move in order to allow more pressure oil to flow to outlets (10) and (18) to the service brakes. This results in a higher oil pressure in cavities (15) and (23). A higher oil pressure is necessary in order to maintain the balance of valve spools (8) and (16)