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Rear wheel cylinder - removal, overhaul and refitting
Braking system / Rear wheel cylinder - removal, overhaul and refitting


Note: Before starting work, check on the availability of parts (overhaul kit of seals). Also bear in mind that if the brake shoes have been contaminated by fluid leaking from the wheel cylinder, they must be renewed. In principle, the shoes on BOTH sides of the vehicle must be renewed, even if they are only contaminated on one side.

Removal
1 Remove the brake drum as described in Section 5.

2 Minimise fluid loss either by removing the master cylinder reservoir cap, and then tightening it down onto a piece of polythene to obtain an airtight seal, or by using a brake hose clamp, a G-clamp, or similar tool, to clamp the flexible hose at the nearest convenient point to the wheel cylinder.

3 Pull the brake shoes apart at their top ends, so that they are just clear of the wheel cylinder. The automatic adjuster will hold the shoes in this position, so that the cylinder can be withdrawn.

4 Wipe away all traces of dirt around the hydraulic union at the rear of the wheel cylinder, then undo the union nut.

5 Unscrew the two bolts securing the wheel cylinder to the backplate (see illustration).

7.5 Bolts securing the wheel cylinder to the backplate. Hydraulic union nut
7.5 Bolts securing the wheel cylinder to the backplate. Hydraulic union nut and bleed screw cover are also visible

6 Withdraw the wheel cylinder from the backplate so that it is clear of the brake shoes. Plug the open hydraulic unions, to prevent the entry of dirt, and to minimise further fluid loss whilst the cylinder is detached.

Overhaul
7 Clean the external surfaces of the cylinder, and unscrew the bleed screw.

8 Carefully prise off the dust cover from each end of the cylinder.

9 Tap the wheel cylinder on a block of wood to eject the pistons and seals, keeping them identified for location. Finally remove the spring.

10 Clean the pistons and the cylinder by washing in methylated spirit or fresh hydraulic fluid. Do not use petrol, paraffin or any other mineral-based fluid. Remove and discard the old seals, noting which way round they are fitted.

11 Examine the surfaces of the pistons and the cylinder bores, and look for any signs of rust or scoring. If such damage is evident, the complete wheel cylinder must be renewed.

12 Reassemble by lubricating the first piston in clean hydraulic fluid, then manipulating a new seal into position, so that its raised lip faces away from the brake shoe bearing face of the piston.

13 Insert the piston into the cylinder. As the seal enters the bore, twist the piston back and forth so that the seal lip is not trapped.

14 Insert the spring, then refit the remaining piston and seal, again making sure that the seal lip is not trapped as it enters the bore.

15 Fit new dust covers to the grooves in the pistons and wheel cylinder body.

16 Refit the bleed screw.

Refitting
17 Wipe clean the backplate, and remove the plug from the end of the hydraulic pipe. Fit the cylinder onto the backplate, and screw in the hydraulic union nut by hand, being careful not to cross-thread it.

18 Tighten the mounting bolts, then fully tighten the hydraulic union nut.

19 Retract the automatic brake adjuster mechanism, so that the brake shoes engage with the pistons of the wheel cylinder. To do this, prise the shoes apart slightly, turn the automatic adjuster to its minimum position, and release the shoes.

20 Remove the clamp from the flexible brake hose, or the polythene from the master cylinder (as applicable).

21 Refit the brake drum with reference to Section 5.

22 Bleed the brake hydraulic system as described in Section 15. Providing suitable precautions were taken to minimise loss of fluid, it should only be necessary to bleed the relevant rear brake.

23 Test the brakes carefully before returning the vehicle to normal service.


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