FRAME GP-MAIN 1200132 - Caterpillar

Location of Park Brake Control and Panel indicators. 1. Low brake pressure indicator.2. Park brake engaged indicator.3. Park brake control.
Location of Pressure Switch (5) for Park Brake Engaged Indicator.
Location of Check Valve (40)Service Brake Valve
The service brake valve is mounted on the cab floor plate.
Location of Service Brake ValveThe valve has two main functions: 1. It controls pump flow to the tractor and trailer braking system accumulators. The valve connects system pressure to the primary pumps load sensing port when the system pressure falls to 15500 kPa (2250 psi). This brings the primary pumps on load to re-charge the systems. When system pressure reaches 17500 kPa (2540 psi) the valve connects the primary pumps load sensing signal to tank and the pumps go off load.2. It controls the flow of oil from the accumulators to the brake groups when a brake application is made.All valve ports are clearly marked with a number which identifies the function of that port.Port "1" is the inlet port from the hydraulic pump.Port "2" is the load sensing port.Port "2.1" is for the trailer brake accumulators.Port "2.2" is for the tractor brake accumulators.Port "2.3" is not used.Port "2.4" is for the pressure switch which activates the low brake oil pressure indicator.Port "5" is for return oil.Port "6.1" is for the tractor brake circuit.Port "6.2" is for the trailer brake circuit.Port "6.3" is for the stop lights pressure switch.Port "6.4" is not used.
Service Brake ValveLoad Sensing System
Hydraulic oil is supplied to the brake control valve (25) through line (22). The oil flows through a restrictor (46) within the valve, which prevents rapid charging of the system, and overcomes a spring loaded check valve (47). The resulting oil pressure flows through a double ball valve (48) to lines (6) and (19) to charge the accumulators (7) and (18).Oil flows through line (49) to spring loaded pressure valve (51) which meters system pressure to load sense line (27). Oil also flows through line (50) to the top of the pressure valve.When the system is fully charged, oil pressure in line (22) overcomes the spring pressure in pressure valve (51) and diverts load sense line pressure to return through line (26) which reduces pressure in load sense line (27) causing the primary pumps to reduce output. The reduced pressure in line (22) allows the higher pressure in accumulator lines (6) and (19) to close the check valve (47) with the assistance of the spring, which retains system pressure in the braking system.Because line (50) is on the high pressure side of check valve (47) the higher system pressure maintains the pressure valve (51) in the discharge position and there is no load sense signal to the primary pumps (unless there is a demand from the steering or body hoist systems). The primary pumps reduce output to standby pressure.Repeated applications of the service brake reduces pressure in lines (6) and (19) which reduces pressure in line (50). This allows the spring in pressure valve (51) to move the