Brake servo operational tests

As the pressure, created by depressing the brake pedal, is insufficient to apply the brakes, some kind of power assistance is required. The most common method is for engine inlet manifold's negative pressure (vacuum) to be used by the brake servo (referred to also as a ‘booster’) to multiply the force applied to the brake fluid by the brake pedal. The servo tends to be a large circular canister to which the brake master cylinder is bolted (as shown). While these cannot be overhauled by a typical DIYER, there are a number of checks that you can conduct to verify that they are working properly.

The basic test involves pumping the brake pedal to remove any residual vacuum within the servo. Apply a constant force on the pedal and start the engine; the pedal should sink slightly. This indicates that vacuum assistance is being created and any problem is likely not to be related to the servo.

Some issues can be caused by the removable servo non-return valve (shown above attached to the unit). Its purpose is to maintain vacuum within the servo after the engine has stopped running and to prevent engine oil vapour from damaging the diaphragms within the servo. To test it, install a hand-held vacuum tester, by fitting a temporary T-piece into the supply line. Start the engine and note the vacuum reading once it steadies. Turn the engine off and ensure that the reading is maintained for at least five minutes. Remove the one-way valve and connect the gauge to the servo side and repeat the test. Any issues point to a failed valve, or a perished rubber grommet.