Booster takes pedal to the floor

I have the Corvette master with the 7" dual diaphragm booster from Speedway.

With the engine not running I have a pretty good pedal. Following trouble shooting instructions, I pump the pedal and hold it down. Then start the engine. The pedal is supposed to drop about 1/4". Mine goes all the way to the floor - 1.5" - 2". Can't find anything that explains this problem.

Any Ideas?

DJ3100 said:

I have the Corvette master with the 7" dual diaphragm booster from Speedway.

With the engine not running I have a pretty good pedal. Following trouble shooting instructions, I pump the pedal and hold it down. Then start the engine. The pedal is supposed to drop about 1/4". Mine goes all the way to the floor - 1.5" - 2". Can't find anything that explains this problem.

Any Ideas?

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Pedal should get real hard with 0 vac from just pumping pedal with engine off...with the pedal held down and starting the motor you will invoke full and I mean full pedal travel...could be close to floor...question is where does pedal actual hold the brakes with the engine running...if pedal feels normal I don't think I'd worry about the "maximum" travel unless it's to the floor....most power brakes actually stop the car far from maximum travel...old rule of thumb...if you can put your toe under the brake pedal at full travel, it's about right.....

Sarge,

Thanks. I don't actually know the answer. I haven't looked at the brakes when the engine was running. I'll have to check it out tomorrow. Maybe they actually are OK!?

I believe he is right brakes are probably well beyond locked up

DJ3100 said:

Sarge,

Thanks. I don't actually know the answer. I haven't looked at the brakes when the engine was running. I'll have to check it out tomorrow. Maybe they actually are OK!?

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I have used that same master cylinder / booster on three different projects and they have all acted just like that. If your pedal is solid when the engine is off and doesn't fade while holding it, you will have brakes. Is your system 4 wheel disc or disc/drum? If its disc/drum you may be able to adjust your rear drums a little tighter and get the pedal up a little.

I would drive it and see how it acts. I bet you'll slide the tires before the pedal hits the floor.

Good luck!

I-R-POPS said:

I have used that same master cylinder / booster on three different projects and they have all acted just like that. If your pedal is solid when the engine is off and doesn't fade while holding it, you will have brakes. Is your system 4 wheel disc or disc/drum? If its disc/drum you may be able to adjust your rear drums a little tighter and get the pedal up a little.

I would drive it and see how it acts. I bet you'll slide the tires before the pedal hits the floor.

Good luck!

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Wow that's good to know. I have the disc/disc. The rears are the GM adjustable parking brake ones that are a bear to adjust. I think they are both adjusted properly, now. So maybe there's a drive in the near future.

Keep it under 100 mph for the first couple of miles.

Ahh good info. I have that same setup and was having trouble with it before shutting it down for hot weather. Will be back to it soon.

DJ3100 said:

Wow that's good to know. I have the disc/disc. The rears are the GM adjustable parking brake ones that are a bear to adjust. I think they are both adjusted properly, now. So maybe there's a drive in the near future.

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You sound like you have the same system I have on my hot rod, mine is a Speedway disc conversion on the rear with those godawful handbrake calipers....

I tried a 1" GM MC but it would not work, turned out to be disc/drum MC.

I then fitted a Wilwood single 7/8" MC and all came good.

I don't like having a single circuit brake system so I have bought a 1" disc/disc Corvette MC to fit some day, no power assist though. I'm thinking this would push far too much fluid to give a good stop though.

Thanks for the help everyone.

I got the wife to step on the pedal with the engine running. At about 1" of pedal travel all 4 discs are tight.

Saved me a bunch of time chasing a problem that isn't there.