776D, 777D and 777D HAA Off-Highway Truck/Tractors Air System and Brakes Service Brakes and Retarder System Leakage - Check Caterpillar

Check all oil lines, hoses, and connections for leaks or for damage. Look for oil on the ground under the machine.

If you are alerted of a brake overstroke condition, check the overstroke switch on the air/hydraulic cylinders in order to find the opened switch. If the actuating pin is pushed out of the housing, the overstroke switch for that air/hydraulic cylinder is open. A leak in the brake hydraulic system causes the air/hydraulic cylinder to travel too far. (This condition is called overstroke.) The actuating pin can be used as a visual means in order to determine if the front system or the rear system is at fault.

If the machine does not have a brake overstroke condition and there is a problem with the brakes, use the FT1097 Displacement Gauge in order to check for system leakage.

Note: Remove air pressure from the air system for the service brakes and for the retarder brake system before you remove the plug from the air/hydraulic cylinder.

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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.

Check The Front Braking System And Check The Rear Braking System For Leakage

Illustration 1	g00916144
Top view (1) Plug on the front air/hydraulic cylinder (2) Plug on the rear air/hydraulic cylinder (A) Hydraulic tank

Make sure that you engage the parking brake.

Note: Make sure that the air pressure is removed from the system before you continue with this procedure.

1. Remove plug (1) from the elbow of the front air/hydraulic cylinder. Remove plug (2) from the elbow of the rear air/hydraulic cylinder.

2. Install a FT1097 Displacement Gauge in the top of the front air/hydraulic cylinder and in the top of the rear air/hydraulic cylinder.

3. Start the engine and allow the air system pressure to rise above 550 kPa (80 psi).

4. Activate the service brakes or the retarder. Push down the rod of the displacement gauge until the rod contacts the plate in the air/hydraulic cylinder. Measure the extension of the rod.

5. After approximately 10 minutes, push down on the rod again. Measure the distance. A decrease in the extension length of more than 3.20 mm (0.125 inch) during the 10 minute check is an indication of leakage. This check determines if the leakage is in the front brake system or in the rear brake system.

Note: Make sure that you remove the air pressure from the system before you remove the gauge and before you install plugs (1) and (2) .

Check For Leakage In The Air/Hydraulic Cylinder

Rear Service and Parking Brakes

Illustration 2	g00647289
Rear Slack Adjuster (3) Fitting (4) Hose

1. In order to check for leakage of the air/hydraulic cylinder, disconnect hose (4) from the slack adjuster and install a 6V-9511 Face Seal Plug in fitting (3) .

2. Install the FT1097 Displacement Gauge. Start the engine and allow the air system pressure to rise above 550 kPa (80 psi).

3. Activate the service brakes or the retarder. Push down the rod of the displacement gauge until the rod contacts the plate in the air/hydraulic cylinder. Measure the extension of the rod.

4. After approximately 10 minutes, push down on the rod again. Measure the distance. A decrease in the extension length of more than 3.20 mm (0.125 inch) during the 10 minute check is an indication of leakage.

5. If no leakage is indicated, see "Brake Piston Seal Leakage Check".

6. If leakage is indicated, check all hose connections for leaks. Also, check the outside of the air/hydraulic cylinder for leaks. If no external leaks are found, check the air/hydraulic cylinder piston seals and the makeup valve for wear or for damage.

Service Brake (Front Oil Cooled)

if equipped

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Illustration 3	g00647290
Front slack adjuster (5) Fitting (6) Hose

1. In order to check for leakage of the air/hydraulic cylinder, disconnect hose (6) from the slack adjuster and install a 6V-9511 Face Seal Plug in fitting (5) .

2. Install the FT1097 Displacement Gauge. Start the engine and allow the air system pressure to rise above 550 kPa (80 psi).

3. Activate the service brakes or the retarder. Push down the rod of the displacement gauge until the rod contacts the plate in the air/hydraulic cylinder. Measure the extension of the rod.

4. After approximately 10 minutes, push down on the rod again. Measure the distance. A decrease in the extension length of more than 3.20 mm (0.125 inch) during the 10 minute check is an indication of leakage.

5. If no leakage is indicated, see "Brake Piston Seal Leakage Check".

6. If leakage is indicated, check all hose connections for leaks. Also, check the outside of the air/hydraulic cylinder for leaks. If no external leaks are found, check the air/hydraulic cylinder piston seals and the makeup valve for wear or for damage.

Front Caliper Service Brakes

1. Oil is sent from the front air/hydraulic cylinder to a tee. Oil flows from the tee to the front caliper service brakes. In order to check for leakage of the air/hydraulic cylinder, disconnect the hose that supplies oil to the tee. This tee is located behind the right front wheel. Install a 6V-9509 Face Seal Plug in the fitting on the hose.

2. Install the FT1097 Displacement Gauge. Start the engine and allow the air system pressure to rise above 550 kPa (80 psi).

3. Activate the service brakes or the retarder. Push down the rod of the displacement gauge until the rod contacts the plate in the air/hydraulic cylinder. Measure the extension of the rod.

4. After approximately 10 minutes, push down on the rod again. Measure the distance. A decrease in the extension length of more than 3.20 mm (0.125 inch) during the 10 minute check is an indication of leakage.

5. If no leakage is indicated, check all hoses and connections from the tee to the brakes for leakage. Check the calipers for leakage.

6. If leakage is indicated, check all hose connections for leaks. Also, check the outside of the air/hydraulic cylinder for leaks. If no external leaks are found, check the air/hydraulic cylinder piston seals and the makeup valve for wear or for damage.

Brake Piston Seal Leakage Check

Note: This check is only for the rear brakes and for the front service brake system, if equipped.

Remove the air pressure from the air system for the service brakes and for the retarder brake system before you remove the plug from the rear air/hydraulic cylinder.

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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.

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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.

Illustration 4	g00916253
(1) Plug on the front air/hydraulic cylinder (2) Plug on the rear air/hydraulic cylinder (A) Hydraulic tank

Note: Make sure that the air pressure is removed from the system before you continue with this procedure.

1. Remove plugs (1) and (2) from the air/hydraulic cylinders. Install an FT1097 Displacement Gauge .

2. Start the engine. Allow the air pressure to reach the cutout pressure. Keep the engine in operation during this procedure to ensure correct air pressure.

3. Move the parking brake control valve to the OFF position.

4. Activate the service brake or the retarder.

5. Push down the rod of the FT1097 Displacement Gauge until the rod contacts the plate in the air/hydraulic cylinder.

6. Measure the length of the rod.

7. After the brakes have been engaged approximately 10 minutes, push down on the rod again. Measure the length of the rod.

8. A decrease in the gauge length of more than 3.20 mm (0.125 inch) during the 10 minute check is an indication of leakage.

9. If you get the same gauge length both times, there is no leakage.

10. If you determine that there is leakage, refer to "Isolating The Leakage".

Note: Make sure that you remove the air pressure from the system before you remove the gauge and before you install plugs (1) and (2) .

Isolating The Leakage

1. First check for slack adjuster failure. See the Air System and Brakes Systems Operation/Testing And Adjusting, "Slack Adjuster - Test and Adjust".

Note: If the slack adjuster has not failed, then perform Step 2.

2. Disconnect the oil line for the service brake and for the retarder from the left side of the slack adjuster.

3. Install a 6V-9831 Cap Assembly on the rear slack adjuster.

4. Perform Steps 2 through 8 in "Brake Piston Seal Leakage Check".

If the length of the displacement gauge decreases, the oil leakage is in the brake assembly on the right hand side of the machine. If the length of the gauge does not decrease, the oil leakage is in the brake assembly on the left hand side of the machine.

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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.

Note: Make sure that you remove all air from the hydraulic brake system. See the Testing and Adjusting, "Brake System Air - Purge" for the procedure.