Brake pedal sinks when engine started.
Thread Starter
7th Gear
Joined: Jul 2002
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Brake pedal sinks when engine started.
I have a '79 t-top that when I start the engine the break pedal sinks to one inch of floor. I've checked all calipers and lines for leaks. I've replaced the master cylinder. I've bled all lines according to shop manual. I've suspected power booster (there is a hiss when the pedal goes down)but others say no. The booster does hold vaceum when engine is shut down, according with the test I perform when disconnecting vac, hose. Any help. Thanks.
Le Mans Master
Joined: Jun 2002
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From: I can walk to MA
Re: Brake pedal sinks when engine started. (jack5995)
I can't solve your problem - but Welcome to the Corvette Forum.
You will get loads of help - as people read the post. So sit tight - read on,
and get some learnin'.
Welcome :seeya
You will get loads of help - as people read the post. So sit tight - read on,
and get some learnin'.
Welcome :seeya
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Re: Brake pedal sinks when engine started. (jack5995)
First...Welcome to our world!! You're going to have fun here!
The power booster sounds like it is working correctly. You are actually performing a function test of it. With all vacuum exhausted from it the pedal should be relatively hard and solid. Then when you crank it up, the added vaccum assist "sucks" the pedal further to the floor, indicating a properly functioning booster. They are really pretty simple, if it works as described, it is probably OK.
Not sure if you know this, but the way a booster works is that when you crank it you have vacuum on both sides of diaphragm inside and it equalizes. When you depress pedal just a little bit, you are actually opening a small checkvalve in the end of the booster to allow atmospheric pressure to enter the "backside" of the diaphragm. Hence the "hiss" noise. The difference in having atmosherric pressure on "your side" of the diaphragm, and vacuum on the other side is what makes it easier to push the pedal and makes power brakes. 20" of vacuum on a 20-30 square inch or so booster is a huge force!! That's why when you add a big cam with less vacuum, the power brakes don't work as well, no vacuum on the other side of that diaphragm as compared to what you had "pulling" it. There are even some small ones with 2 diaphragms inside to increase power.
It is often easy to compress the springs in the master cylinder after you have actually applied full brake effort to calipers. Especially with power brakes, Some master cylinders are really soft in that regard. Drive a late 90's Chevy pickup and you will see what I mean...you can push that pedal almost clear to the floor long after it has actually developed full braking power. GM greatly increased pedal feel on the new series of pickups due to lots of complaints. Ford on the other hand has always kept a pretty solid feeling pedal.
How does it actually stop? I'm guessing it stops hard LONG before th epedal goes that far? If not, then there is most likely still an air pocket in there somewhere. Te best advice I can give on bleeding brakes is to go SLOW. Most folks pump the pedal like crazy and it often aerates things worse.
Good luck!
JIM
The power booster sounds like it is working correctly. You are actually performing a function test of it. With all vacuum exhausted from it the pedal should be relatively hard and solid. Then when you crank it up, the added vaccum assist "sucks" the pedal further to the floor, indicating a properly functioning booster. They are really pretty simple, if it works as described, it is probably OK.
Not sure if you know this, but the way a booster works is that when you crank it you have vacuum on both sides of diaphragm inside and it equalizes. When you depress pedal just a little bit, you are actually opening a small checkvalve in the end of the booster to allow atmospheric pressure to enter the "backside" of the diaphragm. Hence the "hiss" noise. The difference in having atmosherric pressure on "your side" of the diaphragm, and vacuum on the other side is what makes it easier to push the pedal and makes power brakes. 20" of vacuum on a 20-30 square inch or so booster is a huge force!! That's why when you add a big cam with less vacuum, the power brakes don't work as well, no vacuum on the other side of that diaphragm as compared to what you had "pulling" it. There are even some small ones with 2 diaphragms inside to increase power.
It is often easy to compress the springs in the master cylinder after you have actually applied full brake effort to calipers. Especially with power brakes, Some master cylinders are really soft in that regard. Drive a late 90's Chevy pickup and you will see what I mean...you can push that pedal almost clear to the floor long after it has actually developed full braking power. GM greatly increased pedal feel on the new series of pickups due to lots of complaints. Ford on the other hand has always kept a pretty solid feeling pedal.
How does it actually stop? I'm guessing it stops hard LONG before th epedal goes that far? If not, then there is most likely still an air pocket in there somewhere. Te best advice I can give on bleeding brakes is to go SLOW. Most folks pump the pedal like crazy and it often aerates things worse.
Good luck!
JIM
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Re: Brake pedal sinks when engine started. (jack5995)
:confused: it does this by itself, or is your foot on it?
I have a '79 t-top that when I start the engine the break pedal sinks to one inch of floor. I've checked all calipers and lines for leaks. I've replaced the master cylinder. I've bled all lines according to shop manual. I've suspected power booster (there is a hiss when the pedal goes down)but others say no. The booster does hold vaceum when engine is shut down, according with the test I perform when disconnecting vac, hose. Any help. Thanks.
