The Power Booster: Air Valve

By Norman Nock

Originally published in the Austin-Healey Magazine, December 1994

If the little air valves on Healeys equipped with Girling power brakes are damaged, very strange things happen—brakes will lock on when the engine is started, or the brake pedal will be pushed back at you when you apply the brakes. Power brakes are available as an option on Mk II 3000s.

This little valve is not available in a repair kit. Make sure the valve you take out is not lost or damaged. To locate a new valve is difficult.

Figure I shows what they do: when the brakes are in a static position—at rest with no hydraulic pressure applied via of these small air valves—a vacuum is supplied to both sides of the vacuum piston. When the brakes are applied, the valves change position allowing air to pressure to one side of the vacuum piston. Both valves will close when a balance is reached within the servo, and the brakes will be held on.

When the brake pedal is released, the vacuum valve will open and the air pressure inside will be removed, leaving a vacuum on each side of the vacuum piston. They have now released the pressure off the brakes. The operation of these air valves is very critical to the operation of your power brakes. Therefore be sure to check the condition of these air valves and replace them if needed.

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