CYLINDER HEAD SERVICE ASSEM. 8M0694 - Caterpillar

388-6321 Brake Control Valve (Service)
The service brake control valve is located on the front right side, and under the operator platform.A mechanical link connects the two brake pedals to each other. When a brake pedal is depressed, a lever and cam assembly actuates the service brake control valve. The control valve is a fully split modulating valve with two independent output pressures.
Illustration 1 g03390691
Service Brake Control Valve for the 834K Wheel Dozer and the 988K Wheel Loader
(A1) Upper supply ports from the brake accumulator charging valve. (T1) Upper return ports to the hydraulic tank. (P1) Upper supply ports to the service brakes.
(A2) Lower supply ports from the brake accumulator charging valve.
(T2) Lower return ports to the hydraulic tank.
(P1) Lower supply ports to the service brakes.
Illustration 2 g03390692
View of the Service Brake Control Valve for the 834K Wheel Dozer and the 988K Wheel Loader
(1) Piston. (2) Spring. (3) Spring. (4) Retainer. (5) Ball. (6) Spacer. (7) Upper valve spool. (8) Outlet to rear service brakes. (9) Outlet to hydraulic oil tank. (10) Passage. (11) Supply from accumulator for the rear service brakes. (12) Drain port to tank. (13) Upper valve spool passage. (14) Cavity. (15) Lower valve spool. (16) Plug. (17) Outlet to front service brakes. (18) Passage. (19) Lower valve spool passage. (20) Supply from accumulator for the front service brakes. (21) Cavity. (22) Spring.
The service brake control valve modulates the flow of pressurized oil. The flow of pressurized oil is modulated between the brake accumulators and 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.Applying one of the brake pedals will cause a roller arm in the foot brake control to push piston (1). Piston (1) applies a force on springs (2) and (3). The force of springs (2) and (3) moves retainer (4) and ball (5). Movement of retainer (4) and ball (5) causes upper valve spool (7) to move away from the seat in spacer (6).The movement of upper valve spool (7) causes uses the movement of lower valve spool (15) and the compression of spring (22). When valve spools (7) and (15) move, oil flow through passages (10), (12), and (18) to the hydraulic oil tank is blocked.This allows pressurized oil from accumulator ports (11) and (20) to flow through passages (10), (13), (18), and (19). The oil then flows through passages (8) and (17) in order to apply the service brakes. At the same time, pressurized oil flows into cavities (14) and (21), and through passages (13) and (19). The oil pressure at the service brakes has the same pressure as the oil in cavities (14) and (21).Oil pressure in cavity (21) and the force of spring (22) act to balance lower valve spool (15) against the force of the pressure in cavity (14). In the same manner, pressure in cavity (14) creates a force against the bottom of upper