770G and 772G Off-Highway Truck (OEM) Hydraulic and Braking System: Slack Adjuster - Test and Adjust

Table 1

Required Tools
Part Number	Description	Required Amount
198-4240	Digital Pressure Indicator	1
5P-1404	O-Ring Adapter	2
6V-3965	Fitting As	2
8S-2263	Spring Tester	1

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.

When a slack adjuster strokes correctly, there is always a small amount of pressure on the piston in the service brakes after the brakes are released.

Use the following procedure in order to check for a failure in a slack adjuster.

(1) LH pressure tap(2) RH pressure tap



1. Remove the dust caps from pressure taps (1) and (2) of the rear brake slack adjusters.

Rear brake housing (P) Purge valve for parking brakes(S) Purge valve for service brakes

Note: Identification letters are embossed on the purge valves at the top of the brake housing. The purge valve for the parking brake is labeled with a "P". The purge valve for the service brake is labeled with an "S".


2. Remove purge valve "S" on both sides of the machine. Use the 5P-1404 O-Ring Adapter in order to install a 6V-3965 Fitting As if a fitting is not already installed on the machine.

3. Start the machine.

4. Depress the service brake pedal in order to engage the brake.

5. Use 198-4240 Digital Pressure Indicator in order to check the pressure at test port (1), test port (2), and each of the 6V-3965 Fitting As on the rear brakes. The pressure should be equal at each location.

6. Release the brakes.

7. There should be residual pressure at each 6V-3965 Fitting As when the service brakes are released. The oil pressure should be 127 kPa (18 psi).Note: Low residual pressure indicates a failed slack adjuster. High residual pressure may indicate a failed slack adjuster or warped brake discs. Rotate the wheel and check for pressure fluctuations in order to determine if the brake discs are warped. The brake discs are probably warped if the pressure fluctuates. If the brake discs are warped, the brake discs should be replaced.

8. If a problem is indicated, remove the slack adjuster. Refer to the procedure in "Check of Small Piston Movement".

Check of Small Piston Movement

Check of small piston movement

1. Remove the piston from the housing for the slack adjuster.

2. Install a 6.35 mm (0.250 inch) diameter by 31.8 mm (1.25 inch) long rod into the hole of the piston, as shown.

3. Use an 8S-2263 Spring Tester in order to check the movement of the small piston inside the piston.

4. Put the piston in the tester. Apply force on the rod, as shown.

5. Apply a force of 220 N (50 lb) on the rod. If the small piston does not move, replace the piston with a new piston.

6. If the slack adjuster is not damaged, check for damaged oil seals and for brake oil leakage in the brake piston seals.

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.

Note: Purge the brake oil circuit after you check the piston or you replace the piston. Refer to Testing and Adjusting, "Brake System Air - Purge".

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