784C and 785C Off-Highway Truck/Tractors Power Train Caterpillar

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Personal injury or death can result from sudden machine movement. Sudden movement of the machine or release of air pressure or oil pressure can cause injury to persons on or near the machine. To help prevent possible injury before testing and adjusting any air system or hydraulic system, perform the procedure in the Testing and Adjusting, "General Information" section.

This test will show the following conditions if the conditions exist:

• The parking brake release pump section is good.

• The relief valve (3) is correctly set.

Procedure

Illustration 1	g00337540
(E) Location for pressure tap "PMP"

Illustration 2	g00485425
(N) Location for pressure tap

Illustration 3	g00281783
Parking and secondary brake valve (1) Cap (2) Nut (3) Relief valve for parking brake release system

1. Remove the plug at pressure tap (E) or (N). Install a nipple at the location of the removed plug.

2. Use a coupler and a hose to connect 8T-0857 Pressure Gauge to the nipple.

3. Start the engine while the parking brakes are engaged. Run the engine at the low idle rpm with the transmission in NEUTRAL-1. The pressure on the gauge must be 4690 ± 200 kPa (680 ± 29 psi).

4. When this test is complete, stop the engine and remove the test equipment.

Problem: The oil pressure is too low.

Probable Cause

• Failure of the parking brake release pump section

• The setting of relief valve (3) is too low. Remove cap (1) and loosen nut (2). Turn the threaded rod clockwise in order to increase the pressure. One turn increases the setting approximately 690 kPa (100 psi).

• Leakage in the parking brakes.

Problem: The oil pressure is too high.

Probable Cause

The setting of relief valve (3) is too high. Remove cap (1) and loosen nut (2). Turn the threaded rod counterclockwise in order to decrease the pressure. One turn decreases the setting approximately 690 kPa (100 psi).