By Norman Nock
Originally published in the Austin-Healey Magazine, December 1990
The Girling vacuum brake servo is fitted to all BJ8s and as an optional extra on BT7s # 17551 -25314. When fitted, the master cylinder has a larger bore of 7/8 inch.
The power assist for these brakes is obtained by using intake manifold vacuum and atmospheric pressure. The vacuum is created by piston travel and is the same vacuum that draws air and fuel from the carburetors. A vacuum supply line connects the servo to the intake manifold.
If the vacuum to the power brakes is lost, the result would be about a 30-40 percent drop in the available power assist and extra foot pressure would be required to stop your Austin-Healey. A vacuum check valve is installed on the booster that should maintain a residual vacuum after the engine has stopped. This residual vacuum will give about three applications of power assist to the brakes.
To check that your brake system has a working vacuum check valve, turn off the ignition and coast. Operate the brakes. If you have regular stopping power, the valve is working. If you have lost 30 to 40 percent of your braking power, the vacuum check valve is defective and should be replaced.
The correct valve that bolts to the booster is not available, but an inline check valve is available and could be fitted in the vacuum supply line next to the booster.
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