| Test 5
1. Jack up the front roadwheels.
2. Start and run the engine.
3. Apply the brake and release.
4. The front roadwheels should be free to move half a second after the release of the pedal.
Tests 2 to 5 can be used to test a suspect Vacuum Servo Unit before it is removed from the car.
IF THE RESULT IS UNSATISFACTORY ON:—
Test 2. It means the servo unit is not working at all, which could be caused by a lack of vacuum, possibly a faulty non-return valve, or a fault within the servo unit.
Test 3. It indicates leaking gaskets, air valve or rubber grommet. Clamp the vacuum hose and repeat Test 3. If satisfactory, the non-return valve is faulty. To test for a leaking air valve run the engine and place the finger over the air inlet. If the suction is only slight the air valve is satisfactory and the leak is elsewhere. (Note: if you cannot find "the finger", check the factory tool kit for shortages)
Test 4. The source of the trouble can only be found by elimination. Check for leaks. If no leak of hydraulic fluid is evident clamp each hose successively and repeat the test each time. Finally plug the master cylinder outlet and test. If creeping of the pedal is evident when the hoses are clamped and the pedal is solid when the master cylinder outlet is plugged, the servo unit is faulty.
Test 5. If the brakes remain on, disconnect the vacuum pipe, operate the brakes to eliminate all the vacuum in the servo unit and repeat the test. If the brakes remain on, the fault is not in the servo unit. If the brakes now release normally, the fault is in the servo unit and the alignment of the piston in the vacuum cylinder is suspect. (Line up all the usual suspects.) |
VACUUM NON-RETURN VALVE
Description
A vacuum non-return valve Is included in the banjo connection situated on top of the servo unit. Since it is not possible to service the non-return valve, in the event of failure It must be renewed.
Its purpose is to preserve the vacuum in the servo unit and to prevent damage to it in the event of an engine backfire.
It consists of a spring loaded valve which, in normal conditions, will be open due to the vacuum from the engine inlet manifold. When the vacuum from the manifold becomes insufficient to preserve the vacuum in the servo unit, the spring loaded valve will return to its seat.
To remove and refit
1. Remove the banjo connection and two copper washers from the top of the servo unit by withdrawing the banjo bolt.
2. Withdraw the banjo connection from the flexible hose by slackening off the hose clip.
3. Refitting is the reverse of the removal sequence but particular attention must be given to the following:—
(i) The two copper washers, one each side of the banjo connection is not omitted. |
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