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Brake booster check

87 Porsche 944, Girling booster

I have the brake master cylinder off. What checks can be done on the booster like that, or by removal of the booster?

The cylinder is bad, but I don’t know how to distinguish symptoms like brake clutch pedal stuck on the floor being from cylinder or booster. Can’t start the car yet because of other work.

I’ll defer to others but the only time I’ve had a brake pedal stick to the floor it was the booster in my 59 Pontiac. I see no way a master could cause the pedal to stick. Sink yes, but not stick.

@Bing oh no, confusion ahead : then I ought to mention the brake line also had a hole, and that the clutch pedal - not the brake -'as I erroneously wrote - would need to be pulled up with my foot to work.

That mess what I was hoping to avoid by focusing on the booster as it sits now, but if anyone can stand it, I appreciate the comments.

A test bench for a vacuum brake booster is beyond the scope of DIY auto repairs…

Count me out, the only clutch I ever had used a cable. If it didn’t come up, you pulled on the cable.

THe return spring for the clutch peddle is off or broken.

Leave the booster alone, if it worked fine before.


Yosemite That’s what I was thinking. Every clutch I have owned or driven had a return spring.

@Yosemite I will find out about the spring later - that’s for another thread entirely.

Without specialized equipment it’s impossible to test the diaphragm or the associated internal valves of the booster. The only thing you can check on the booster with the master cylinder removed is that the movement of the power rod that goes through its center and connects to the master cylinder is operating unrestricted. You might want to test for that since your pedal was sticking to the floor, however I’d bet the sticking is caused by the master cylinder.

The way the master cylinder is made, it consists of two pistons in a chamber, the forward surface of the pistons being what transfers the pressure to the calipers, the back surface of the pistons being what draws fresh fluid from the vented reservoir and keeps the MC full. If the piston seals, which are rubber, are deteriorated, they can become stuck in the cylinder and prevent the piston from withdrawing.

Now c’mon, junior, you’ve visited before. You know we can better help with if we know the year, make, model and mileage. Are the answers are…? We’re always happy to help, but we can better do so if we know something about the vehicle.

@"the same mountainbike"‌ '87 Porsche 944 with Girling booster and cylinder.

I noted above, and should edit the post, it is the clutch pedal, and not the brake pedal, which needed to be pulled back to function… and I am replacing the clutch MC/SC and the hose.