Fiat Uno Manual

Master cylinder - removal, overhaul and refitting
Braking system / Master cylinder - removal, overhaul and refitting


Note: Purchase a repair kit in advance of overhaul.

1 The master cylinder is mounted on the front face of the brake vacuum servo unit (55 and 70 models) or directly to the bulkhead (45 models).

2 Cover the front wings with polythene sheeting or similar material, in case hydraulic fluid spills onto the paintwork of the car during removal of the cylinder.

3 Detach the leads from the terminals on the reservoir cap, then unscrew and remove the cap and float.

4 Unscrew the pipe unions and prise the pipes carefully away from the master cylinder.

Cap the open ends of the pipes and catch any fluid leaking from the master cylinder in a suitable container.

5 Unscrew the mounting nuts and withdraw the master cylinder from the bulkhead or from the servo unit.

6 Clean away all external dirt and tip out the fluid from the reservoir and cylinder body.

7 The fluid reservoirs need not be removed from the master cylinder but if they are, renew the rubber sealing collars when refitting.

8 Grip the master cylinder in a vice, then unscrew and remove the end plug. Catch the coil spring.

9 Using a thin rod, apply pressure to the end of the primary piston then unscrew and remove the two stop bolts and sealing washers.

10 The internal piston assemblies with seals and springs can now be pushed out of the cylinder body. Keep all the components in their originally fitted sequence and note in which direction the seal lips are located.

11 Inspect the surfaces of the piston and cylinder bore. If scoring, corrosion or metal-to-metal rubbing areas are evident, renew the master cylinder complete.

12 If the components are in good condition, discard the oil seals and manipulate the new ones into position, using the fingers only.

13 Refit by reversing the removal operations; apply pressure to the piston ends so that the stop bolts can be fitted, then tighten the end plug. Make sure that the grooves in the pistons engage in the stop bolts.

14 Before refitting the master cylinder to the servo, measure the projection of the servo piston pushrod. When the master cylinder is fitted, there must be a clearance (see A in Fig. 8.7) between the end of the pushrod and the primary piston end face of between 0.825 and 1.025 mm (0.03 and 0.04 in). A depth gauge will be required for these measurements, the reference point being the mating surfaces of the master cylinder and the vacuum servo.

15 Alter the adjusting screw on the servo as necessary and lock it by applying locking fluid to the threads on completion.

16 Bolt the master cylinder to the vacuum servo or bulkhead, then reconnect the pipelines and reservoir cap leads.

17 Bleed the complete hydraulic system, as described in Section 12.

Fig. 8.5 Sectional view of master cylinder (Sec 9)
Fig. 8.5 Sectional view of master cylinder (Sec 9)

1 Cylinder body
2 Spring and cup
3 Inlet from reservoir
4 Secondary piston
5 Seal
6 Fluid outlet to front brakes 7 Spring and cup
8 Inlet from reservoir
9 Primary piston
10 Seal
12 Stop bolts
13 Spacer
14 Springs
15 Seal
16 End plug and fluid outlet to rear brakes

Fig. 8.6 Exploded view of master cylinder (Sec 9)
Fig. 8.6 Exploded view of master cylinder (Sec 9)

1 Cylinder body
2 Secondary piston
3 Primary piston
4 Stop bolt

Fig. 8.7 Sectional view of vacuum servo unit (Sec 9)
Fig. 8.7 Sectional view of vacuum servo unit (Sec 9)


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