Brake Booster

[scarface031]

I can't really find this anywhere on the internet.

I currently have my booster disconnected from the manifold because of this problem.

When I have the brake booster connected, the brake pedal pushes down as soon as I start the car.

Any input on further diagnosis?

What should I do?

[scotty]

I assume that the brakes are working & there is good pedal feel?
With the vacuum pipe connected the brake pedal goes down when you start the car right?
If this happens with your foot on the pedal, this is correct operation. That's how it is supposed to work.
If it goes down with no foot on the brake then you have a pressure imbalance either side of the diaphragm & at rest there should be equal pressure on either side. When the pedal is pressed, the pressure equalising valve gives you vacuum on the cylinder side & atmospheric pressure on the pedal side, which pushes the diaphragm, assisting your manual effort.
Simples!


Scotty, South Australia.
1978 TR7 2.0 LT77
1981 XJS

[HDRider]

I have not had that problem but I agree that there is something seriously wrong with the booster. You can get rebuilt kits from Victoria British. Taking apart the booster is not for the faint of heart though.

The rebuild kits have new seals, diaphram and valves but the plastic piston is not available so if that broke you are out of luck. Victoria used to sell comlete units but they do not seem to have them listed any longer. I expect that you can send your unit out through a local FLAPS to get rebuilt though.

I have a couple of used ones in the back shed if you are anywhere near to San Francisco CA.

Edward Hamer
Petaluma CA

[Beans]

Have to agree with Scotty.

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[jeffremj]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">If this happens with your foot on the pedal, this is correct operation. That's how it is supposed to work.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">I agree, this has always happens on my TRs.

[scarface031]

It drops when I am not even touching the pedal, like they are applying them selves.

[FI Spyder]

I've not had the problem or had the booster apart. A club member with a Dolly Sprint had his apart and apparently there is a big spring inside (which you have to be careful of if taking apart for overhaul). I wonder if the spring was broken if you would get that sort of action.

As it turned out the body of the booster was rusted so wouldn't hold vacuum. He ended up using a overhauled TR6 booster which was bigger so got more assist and there was only a little fettling to get it too work (re, mounting holes).

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[saabfast]

I think there is a diaphragm and the big spring to assist return. If the diaphragm split it might give give your symptoms. Best way is to replace it with an upgraded version from S&S or the like. There is also a non return valve in the pipe or connector on the servo which might have failed, but not sure it would give that result.

I had problem with the master cylinder and replaced with a recon unit which leaked so bit the bullet and replace the servo and M/C with an upgraded one from S&S (possibly from a Sherpa van?). Brakes are good now even with the standard disc/drum setup.

Alan
Saab 9000 Stg 1 (now passed to son for his family car)
Saab 9000 2.3 FPT Auto (now gone that others might live)
Saab 9000 2.3 LPT Auto
Saab 9-5 2.3 Vector Auto Estate
'81 TR7 DHC