GPA20 rear axle - Brake piston

The brake pistons are housed in two lateral cavities of the rear axle housing, concentric with the mating face of each trumpet housing. They comprise a shouldered part and are guided by 3 centring pins force-fitted in the housing. Sealing is provided by two "O" rings fitted in the grooves of the rear axle housing. Each piston is controlled by a master cylinder topped by a tank whose level is kept constant by the residual oil flow from the 17 bar valve. Each piston acts directly on a friction disc with inserted hub, fitted to the trumpet housing input sun gear and a plate centred by pins bearing on the final drive ring gear. When the brake is released, a self-adjusting mechanism ensures minimum clearance between the piston and the brake disc and maintains a constant pedal clearance. The brake discs are lubricated by a flow of oil. This continuous flow from the linkage valve is directed to the brake discs via a pipe and a restrictor union screwed onto the trumpet housing. The oil circulating between the discs and the pistons lubricates the braking surfaces when they are not in use and cools the brakes when they are being used.

Remove the trumpet housing.

Remove the disc (1), the sun gear (2), and the centring pin (3).
Remove the brake plate (1) (inside the trumpet housing).

Remove the screws (3) using a makeshift socket. 

Take the piston (1) out of the housing using a jet of compressed air applied to the union (2)

To facilitate access to the union (2), if necessary, disconnect the 4-speed economy PTO control (if fitted) and turn the union.
Remove the "O" rings (1) and (2) on the housing and discard them.

Check and clean the components. Replace any defective parts. To ensure that the brake self-adjusting system operates correctly, it is essential to centre the right and left-hand differential flanges using a makeshift tool.

Install the piston (1) without seals in the housing (2) and check that the piston slides freely in the bore of the housing and that it fits smoothly onto the centring pins (3). Remove the piston after carrying out this check.
Install spring pins (4) about 8 mm from machined face "F" of the piston (3). Install the new "O" rings (1) and (2) after smearing them with a moderate quantity of miscible grease so that they remain at the bottom of the groove.

Position the piston over the centring pins, then Install the piston in place using a plastic hammer, striking alternate points around the rim. Install the screws (3) after lightly smearing them with Loctite 241 and tighten them to a torque of 10 Nm with a makeshift socket.

Sealing test
If work is carried out on the brake piston and "O" rings, it is necessary to check for leaks.
Check the condition of the supply union fitted on the centre housing.
Install a pressure gauge to the union.
Supply the system with compressed air at approx. 5 bar to ensure that the piston and "O" rings are positioned correctly. Reduce the pressure to 0.3 bar to carry out the test. Close off the pressure relief valve. For one minute, no drop in pressure should be observed on the pressure gauge.
Remove the pressure gauge and reconnect the brake hose.

Reinstall the trumpet housing
Clean the mating faces on the housing and on the trumpet housing with a non-greasy solvent. Apply a bead of silicon resistant to hydrocarbons (Silicomet type) to the inside edge of the housing. Re Install the sun gear (2), the disc (1), and the centring pin (3). Check that the disc slides freely on the sun gear. Reinstall the trumpet housing.

Bleed the main brake system and trailer brake system (if fitted).
- the linkage
- the braking by carrying out a road test
Check tightness:
- between the trumpet housing and the rear axle housing
- of the linkage valve supply hose