Testing
1 With the engine switched off, depress the brake pedal several times. The distance by which the pedal moves should now alter over all applications.

2 Depress the brake pedal fully and hold it down then start the engine. The pedal should be felt to move downward slightly.

3 Hold the pedal depressed with the engine running, switch off the ignition and continue to hold the pedal depressed for 30 seconds during which period the pedal should neither rise nor drop.

4 Start the engine whilst the brake pedal is released, run it for a minute and switch off.

Give several applications of the brake pedal.

The pedal travel should decrease with each application.

5 Failure of the brake pedal to act in the way described will indicate a fault in the servo unit.

6 The servo unit should not be serviced or overhauled beyond the operations described in this Section and in the event of a fault developing, renew the servo complete.

7 Periodically check the condition of the vacuum hose and security of the clips.

8 Renew the hose if necessary.

9 If the servo hose right-angled non-return valve is loose in its sealing grommet, or if the grommet shows evidence of cracking or perishing, renew it. Apply some hydraulic fluid to the rubber to facilitate fitting.

Air filter renewal
10 Although not a specified operation, the air filter through which the pushrod passes at the rear of the servo can become clogged after a high mileage. Disconnect the rod from the pedal, cut the filter diagonally having slipped the dust excluder off the rod. Fit the new filter.

Servo unit removal
11 Remove the master cylinder, as described in Section 12. Disconnect the servo vacuum hose.

12 Working inside the car, disconnect the pushrod from the brake pedal; noting that it is on the lower hole.

13 Unscrew the mounting nuts behind the pedal bracket then withdraw the servo unit into the engine compartment. Remove the gasket.

Servo unit refitting
14 Before fitting a servo unit, check the pushrod dimensions and adjust where possible (see illustration).

14.14 Cross-section of the vacuum servo unit showing pushrod adjustment dimensions

1 Adjustment screw
A = 22.2 to 22.4 mm
B = 86.5 to 89.5 mm

15 Refitting is a reversal of removal, but fit a new gasket and fully tighten the mounting nuts. Note that the pushrod is fitted to the lower hole of the two on the brake pedal. Refer to Section 12 when refitting the master cylinder. Finally, with the brake pedal released, check that the clearance between the stop light switch threaded shank and pedal is 3.5 mm. If necessary loosen the locknuts, adjust the switch and tighten the locknuts.