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Brake Booster Failure Symptoms


Senior Member
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What are symptoms of a malfunctioning booster? Any hints on diagnostic techniques? Disconnect the line to the manifold?

I noticed driving my 1967 BJ8 home over the weekend that it felt like the brakes were being lightly applied during acceleration. The brakes seemed to free up when I lightly tapped the brake. The original booster was sent out and rebuilt about 15 years ago in California while the car was in restoration. It may have been an exchange rather than the original.

I use silicone fluid. The pistons in the calipers are stainless.

If I replace it, is it recommended to switch to a Lockheed version? I don't care about total originality. I just want function. I have an alternator, Pertronix ignition, and 3:54 rear end, so concours is not that important. BTW, I have owned this car for 51 years. Any help would be appreciated.


Great Pumpkin
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Doesn't vacuum keep the booster retracted? If that's the case, you could have a vacuum leak and the vacuum in the canister is too low. I had a problem on my MGB where there was a leak at the fitting where the vacuum line slips into the canister. The rubber seal went bad and it was a pain to find. PJ
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I would assume that the breaking would become harder without the assist of the booster and would not have any affect on acceleration (but what do I know).
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Assuming you have an original Girling servo, during normal running, manifold vacuum is applied equally to both sides of the big piston that powers the boosted pressure to the brakes. When you apply the brakes, ambient air is applied to the outer part of the big piston, and the pressure difference between ambient air pressure and the reduced pressure on the 'underside,' amplified by the greater area of the booster piston to the smaller 'boost' piston applies the power.

One of the reported failures is of the big vaccum/air piston sticking after application, causing pressure to be exerted on the brakes at all times. Applying the brakes may free up this big piston (at least temporarily). This could well be the cause of the problem you're experiencing; the big piston is sealed with a leather seal, which is backed by a rubber tube. The vacuum cylinder is lined with a dry adhesive, and if this wears through it can cause the piston to stick (note this only applies to original Girling servos, aftermarket ones use a diaphragm instead of a piston).

You can test overall functioning of the servo by applying heavy pressure on the brake pedal as the engine is started, if the servo is functioning the pedal will go down an inch or so. If your servo is sucking brake fluid--check the level in your reservoir--you will likely eventually get some white smoke out of the tailpipe.
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