Vacuum servo unit - servicing and testing

Braking system / Vacuum servo unit - servicing and testing

1 Regularly, check that the vacuum hose which runs between the servo unit and the inlet manifold is in good condition and is a tight fit at both ends.

2 If broken or badly clogged, renew the air filter which is located around the brake pedal push rod. Access to this is obtained by disconnecting the pushrod from the cross-shaft or pedal arm, withdrawing the pushrod, dust excluding boot and end cap.

3 If the new filter is cut diagonally from its centre hole, future renewal can be carried out without the need for disconnection of the pushrod.

4 If the efficiency of the servo unit is suspect, it can be checked out in the following way.

5 Run the engine, then switch off the ignition.

Depress the footbrake pedal; the distinctive in-rush of air into the servo should be clearly heard. It should be possible to repeat this operation several times before the vacuum in the system is exhausted.

6 Start the engine and have an assistant apply the footbrake pedal and hold it down.

Disconnect the vacuuum hose from the servo.

There should not be any in-rush of air into the servo through the connecting stub. lf there is, the servo diaphragm is probably faulty. During this test, expect the engine to idle roughly, unless the open end of the hose to the inlet manifold is plugged. Reconnect the hose.

7 With the engine off, depress the brake pedal fully. Start the engine with the brake pedal still depressed; the pedal should be felt to go down fractionally.

8 If the results of these tests are not satisfactory, remove the unit and fit a new one as described in the next Section.