Vacuum servo unit - testing, removal and refitting

Braking system / Vacuum servo unit - testing, removal and refitting

Testing
1 To test the operation of the servo unit, depress the footbrake several times to exhaust the vacuum, then start the engine whilst keeping the pedal firmly depressed. As the engine starts, there should be a noticeable “give” in the brake pedal as the vacuum builds up. Allow the engine to run for at least two minutes, then switch it off. If the brake pedal is now depressed, it should feel normal, but further applications should result in the pedal feeling firmer, with the pedal stroke decreasing with each application.

2 If the servo does not operate as described, first inspect the servo unit check valve as described in Section 13.

3 If the servo unit still fails to operate satisfactorily, the fault lies within the unit itself.

Repairs to the unit are not possible - if faulty, the servo unit must be renewed.

Removal
4 The vacuum servo unit is removed together with the master cylinder, then dismantled on the bench. First disconnect the battery negative terminal. To improve access to the master cylinder, remove the air inlet duct as described in the relevant Part of Chapter 4.

5 Remove the master cylinder reservoir cap, and syphon the hydraulic fluid from the reservoir. Note: Do not syphon the fluid by mouth, as it is poisonous: use a syringe or an old poultry baster. Alternatively, open any convenient bleed screw in the system, and gently pump the brake pedal to expel the fluid through a plastic tube connected to the screw (see Section 2). Disconnect the wiring plug from the brake fluid level sender unit.

6 Wipe clean the area around the brake pipe unions on the side of the master cylinder, and place absorbent rags beneath the pipe unions to catch any surplus fluid. Make a note of the correct fitted position of the unions, then unscrew the union nuts and carefully withdraw the pipes. Plug or tape over the pipe ends and master cylinder orifices, to minimise the loss of brake fluid, and to prevent the entry of dirt into the system. Wash off any spilt fluid immediately with cold water.

7 Working inside the car extract the clip, remove the pin and disconnect the vacuum servo pushrod from the pedal.

8 Working in the engine compartment, carefully ease out the vacuum hose from the servo unit sealing grommet.

9 Unscrew the nuts and remove the washers securing the servo unit to the bulkhead.

Where applicable, note the location of the retaining plate on the upper stud. Remove the assembly from the engine compartment.

1O Slacken and remove the two nuts and washers securing the master cylinder to the vacuum servo unit, then withdraw the master cylinder and recover the rubber O-ring.

Filter renewal
11 If necessary the filter may be removed as follows. Loosen the locknut then unscrew the clevis from the pushrod - to ensure correct refitting note carefully the position of the clevis. Unscrew the locknut.

12 Slide the protective sleeve from the servo unit, then use a hooked instrument to remove the filter and damping washer.

13 Clean the filter location then insert a new filter and damping washer making sure that the slots of each item are offset by 180° from each other.

14 Refit the sleeve followed by the locknut and clevis. After positioning the clevis in its previously noted position, tighten the locknut.

Refitting
15 Remove all traces of dirt from the master cylinder and servo unit mating surfaces, and fit a new O-ring to the groove on the master cylinder body.

16 Fit the master cylinder to the servo unit, ensuring that the servo unit pushrod enters the master cylinder bore centrally. Refit the master cylinder mounting nuts and washers, and tighten them to the specified torque.

17 Refit the assembly to the bulkhead and tighten the mounting nuts. Where applicable, make sure the location plate is located on the upper stud.

18 Ease the vacuum hose back into position in the servo unit, taking care not to displace the sealing grommet.

19 Inside the car connect the pushrod to the pedal with the pin and retain with the clip.

Note: If a new servo unit has been fitted adjust the pushrod as described in Section 11.

20 Wipe clean the brake pipe unions, then refit them to the master cylinder ports and tighten them securely.

21 Refill the master cylinder reservoir with new fluid, and bleed the complete hydraulic system as described in Section 2.

22 Refit the air inlet duct (where removed) and reconnect the battery negative terminal.