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Tuesday, January 22, 2013

770G and 772G Off-Highway Truck (OEM) Hydraulic and Braking System: Parking and Secondary Brake Release System Leakage - Check & Brake System Air - Purge






Parking and Secondary Brake Release System Leakage - Check

An internal oil leak in the brake release system will cause the outlet pressure of the parking brake release pump to decrease. An internal oil leak can also cause too much friction between the brake discs and the plates. This can cause damage to the brake discs and the plates.

  1. Check the brake release system for an internal leak.
    (1) Oil line that is connected to the TCS valve(2) Brake control valve

  1. Disconnect the oil line that is connected to the TCS valve at the parking and secondary brake control valve. The valve is located across from the hydraulic tank on the right side of the frame.

  1. Install a gauge that has a capacity of 6890 kPa (1000 psi) pressure and either a gate valve or a needle valve. The valve and the gauge should be installed between the TCS valve and the brake control valve.Note: The gauge must be connected between the TCS valve and the valve that was installed in step 3. There must not be any oil leaks after the gauge and the valve have been installed.

  1. Start the engine.

  1. Shift the transmission into the NEUTRAL position. This will disengage the parking brake.

  1. Operate the engine at high idle. High idle is 2310 ± 50 rpm.

  1. Check the oil pressure that is shown on the gauge. The oil pressure should be 4689 ± 690 kPa (680 ± 100 psi).Note: If there is any oil leakage in the brake system, the gauge may not show a pressure of 4689 ± 690 kPa (680 ± 100 psi).

  1. Close the valve and look at the gauge. If the pressure has not decreased by 690 kPa (100 psi) after one minute, the brake release system will not be affected by the oil leakage. Perform the procedure that is in the Testing and Adjusting, "Parking Brake Release Pressure - Test" section of this service manual.Note: If the oil pressure in the brake release system decreases by 690 kPa (100 psi) or more after one minute, there is too much oil leakage and each component must be checked.

    Illustration 2g00331295
    (5) Tube

  1. Disconnect tube (5) from the right rear brake assembly.

  1. Connect a gate valve or a needle valve to tube (5) . Install a gauge that has a capacity of 6890 kPa (1000 psi) to the valve and the brake housing.

  1. Start the engine.

  1. Shift the transmission to the NEUTRAL position.

  1. Operate the engine at high idle. High idle is 2310 ± 50 rpm.

  1. Close the valve that is connected to tube (5) . Look at the gauge that is connected to the valve and the brake housing. If the oil pressure that is shown on the gauge decreases by 690 kPa (100 psi) in one minute, there is oil leakage in the brake assembly for the right wheel. If there is no drop in the oil pressure, there is no damage to the circuit for the right rear brake.

  1. Remove the needle valves or the gate valves and remove the gauges. Reconnect tube (5) .

  1. Remove any air from the hydraulic oil brake system.

WARNING: Personal injury or death can result from air in the oil for the brake hydraulic system.
Air in the system can prevent complete brake application and it is possible that the wheels on the machine can not be stopped.Air must not be in the oil for the brake hydraulic system to function properly.
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Brake System Air - Purge

Table 1
Required Tools    
Part Number    Description    Required Amount    
6V-4144    Quick Connect Coupler    1    
235-7104    Hose As    2    
1S-8937    Needle Valve    1    

There are quick disconnects for the hydraulic brakes on each rear wheel. Before you purge air from the oil in the system, the brake system accumulators must be fully charged.

WARNING: Personal injury or death can result from air in the oil for the brake hydraulic system.
Air in the system can prevent complete brake application and it is possible that the wheels on the machine can not be stopped.Air must not be in the oil for the brake hydraulic system to function properly.
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Personal injury or death can result from machine movement.Place blocks in front of and behind the wheels to make sure the machine does not move while the parking brakes are disengaged.
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NOTICE
Care must be taken to ensure that fluids are contained during performance of inspection, maintenance, testing, adjusting, and repair of the product. Be prepared to collect the fluid with suitable containers before opening any compartment or disassembling any component containing fluids.
Refer to Special Publication, NENG2500, "Dealer Service Tool Catalog" for tools and supplies suitable to collect and contain fluids on Cat products.Dispose of all fluids according to local regulations and mandates.
Note: Purge only one wheel at a time.

Service Brakes and Retarder


Rear Brakes


  1. Assemble the following in order to construct a purge hose assembly: one 6V-4144 Coupler, two 235-7104 Hose As, and one 1S-8937 Needle Valve
    Purge hose assembly
    (1) Hose Assembly
    (2) Quick Connect Coupler(3) Needle Valve
    (1) Hose Assembly
    (2) Quick Connect Coupler(4) Quick disconnect for the right rear brake

  1. Connect the 6V-4144 Quick Connect Coupler (2) to the quick disconnect fitting (4) on the rear brakes.
    Note: In order to make sure that the brake accumulators are fully charged, operate the engine at high idle for 20 seconds before each brake application.

  1. Remove the cap from the hydraulic oil tank (5) and insert the other end of the purge hose assembly (1) into the filler neck.

  1. Run the engine at low idle.

  1. Open the 1S-8937 Needle Valve (3) .Opening the valve will allow oil to flow to the hydraulic tank.

  1. Depress the service brake pedal. Hold the brake pedal for approximately 20 seconds. Depressing the pedal will purge the air from the hydraulic brake oil.

  1. Close the 1S-8937 Needle Valve (3) .

  1. Release the service brake pedal.

  1. Disconnect the 6V-4144 Quick Connect Coupler (2) from the quick disconnect fitting (4) on the rear brakes.

  1. Repeat Steps 2 through 9 for the other rear wheel brake.

Front Brakes

Note: In order to make sure that the brake accumulators are fully charged, operate the engine at high idle for 20 seconds before each brake application.

  1. Depress the service brake pedal. Run the engine at low idle.
    View of front service brake caliper(6) Purge valve

  1. Connect a drain hose from purge valve (6) to the hydraulic tank or to a suitable container.

  1. Open purge valve (6) on a front wheel brake housing and purge the brake system.

  1. Repeat Steps 1 through 3 until there is a continuous flow of oil.

  1. Close the purge valve. Release the brake pedal.

  1. Complete Steps 1 through 5 on the other front wheel brake.

  1. Remove the drain hose.


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