Master cylinder leaks down booster all the time!

Does anyone else have this problem? no matter what I do, weather I use a new master, or a rebuilt one, the freaking master leaks down the brake booster. it leaks through the back where it goes into the booster. Anyone else have this problem? How can I fix it?

thanks

Bob

 

The only time I had that problem, on my 71 GS455, was before the master cylinder gave up. The seals were leaking. Then, just before launching at the strip, my brake light came on. I made a pass without brakes essentially. They still worked enough to stop me after the run and drive home, but it was white-knuckle time.

I don't understand why it would happen on a new cylinder though???

 

Lon Bauer on this board had a similar problem and eventually found the culprit. Here's his PM page:


http://v8buick.com/member.php?s=&action=getinfo&userid=62

 

Mine does that also, I've tried 2 different ones and even rebuilt one myself. I blamed it on the silicone fluid.........keep us posted, I am curious to learn what causes this. :bglasses:

 

It seems like the only thing it could be is fluid slipping past the piston's seals. Unless it's coming out of the cover during acceleration, how else would fluid travel from inside the cylinder out through that area???

Maybe the silicone fluid is able to get past the seals even on new cylinders. I'm curious if Bob is using the silicone fluid too.

 

I'll have to check and see what fluid I'm using... its dot3, i know that. Any recommendations on fluid?

 

If you plan on keeping your car, drain your system, flush and install silicone fluid. It will stay clear and water free! The regular brake fluid is alcohol based, absorbs water and is hard on seals.
Silicone brake fluid will lubricate rubber seals and hoses.

Another idea, do you have the correct length input shaft from the power booster? If you have the wrong one, you may be over driving the master cylinder causing leaks.

 

George, how do i know if it is the correct length shaft coming out the booster?

 

I know of at least 2 different lengths. I believe the correct one is about 1-1.25" long.

 

In the service manual I see a picture of a guage, you know, those J tools? Shame it doesn't specify the exact dimensions though. I think there is two different specs b/c the Bendix rod sticks out further than the Delco, I don't know about manual brakes though. :bglasses:

 

I should mention that I'm using a corvette master, from a 70 vette. It looks very similiar to the oval shape GSX style masters, without the bleeder screws. It doesn't leak out of the cap though like the rectangle style did. the mounting is the same though.