Brake pistons and seals of tractor equipped with trumpet housing

The brake pistons (2) are housed in two cavities formed by the centre housing and the differential carrier, concentric with the thrust face of each trumpet housing. The brake pistons are guided by three locating pins (6) force fitted into the centre housing and have an annular face that is submitted to pressure from the master cylinder. The other face has a larger surface area and acts upon a disc that is integral via splines with the input sun gear of the trumpet housing and on the wall of a compartment that also serves as the brake plate. This compartment is centred by locating pins that are force fitted into the trumpet housing which may or not be oil tight depending on the type of assembly. The oil tightness of the pistons is provided by O’rings fitted into the grooves of the centre housing and the differential carriers. Two other seals placed in the angle of each carrier stop the pressure in the piston chamber from communicating with the interior of the centre housing. The shape of the pistons differs according to the options and the type of trumpet housing. Each piston is controlled by a master cylinder the oil level of which is maintained by a residual oil flow from the lubricating circuit. When the brake is released, a minimum clearance is provided between the piston and the disc. The brakes are self-adjusting and maintain a constant brake pedal position. When they are not in use, the discs are lubricated by an oil flow from the lubricating circuit on the right-hand cover. This oil also serves to cool the discs when they are used.

On tractors fitted with trumpet housings with double drive unit, the brake pistons (2) are fitted in the same cylinders as previously mentioned. They are guided by three stop pins (39) (41) mounted free in the housing. Pin (39) fitted in the upper part of the housing has a drilled central port to allow pressure venting from the trumpet housing into the rear axle and breather pipe. The pistons act upon a two-disc system (35) running in an oil bath and integral via splines with hub (32) and the trumpet housing input shaft. The discs are separated by an intermediate plate (38) that is guided by the previously mentioned pins. During braking, the double-disc system is thrust against the centre brake plate by pin (7) on the centre housing.

Parts list
(1) Centre housing (2) Piston (3) O’ring (4) O’ring (5) O’ring (6) Stop pin (7) Pin (32) Hub (35) Disc(s) (38) Intermediate plate (39) Stop pin (drilled) (41) Stop pin

Disassembly and reassembly the brake pistons

Remove the trumpet housing(s) concerned.

Remove the master cylinder brake supply pipe from the union above the centre housing. Progressively drive out piston (2) from housing (1) using a compressed air jet connected to the union (3).

Remove the O’rings (3) (4) fitted on the housing and differential carriers respectively.

- The seals (3) (4) not being mounted on the brake piston, their movement and wear is limited.
- The O’rings (5) fitted between the differential carriers and the centre housing ensure the oil tightness of the piston chambers. If necessary, remove the differential carrier(s) to replace the previously mentioned O’rings.

- If the spacer acting as the brake plate has to be replaced, consult the section concerning the relevant trumpet housing type.
Clean and check the components. Replace any parts found to be defective.
If removed, refit the differential carrier(s) with a new seal.

Checks before reassembly
- Remove the O’rings (3) (4) and place the piston in the housing.
- Check that the piston slides freely in its bore and that there are no stiff spots on stop pins (6) (39) or (41) (depending on the version).

When the check is completed, definitively fit the new seals previously lubricated with transmission oil and position the piston on the stop pins, turning the more numerous lubricating ports upwards.
- To avoid damaging the seals, place the piston in its bore using a locally manufactured tool and with the aid of two bolts "V" of a suitable length that are alternatively and progressively tightened.

Remove the tool. If necessary, complete the fitting by alternatively tapping on the face of the piston using a plastic hammer. Check that there are no fragments of O’rings after fitting.

Hydraulic test 

Positioning of seals
- Use a pressure gauge that has previously been checked for pressure tightness and fit the piston retainer tool onto the housing.

- Using compressed air, pressurize the circuit to approximately 5 bar.
- Disconnect the pressure gauge and push the piston back using a plastic hammer.
- Repeat the above operation several times.

- Pressurize the circuit again to a pressure of approximately 0.3 bar.
- Close the pressure relief-valve.
- Over a period of one minute, no drop in pressure should be seen on the pressure gauge.
- After the test, remove the pressure gauge and take off the retainer tool.

Refit the trumpet housing(s). Purge the main brake system. Carry out a road test of the brake circuit. Check the oil tightness of:
- trumpet housing seals (silicone)
- brake lubricating pipes