No Brakes When Engine Running
Thread Starter
Racer
Joined: Nov 2019
Posts: 336
Likes: 173
From: Canadian living in southern France
No Brakes When Engine Running
I have bled my '76 brakes by gravity, and with a Motive Power and there is no air in it at all.
I have bench bled the master cylinder on the car following the instructions posted here. The front half had no air, the back half had about 10 bubbles about the size you get in soda.
The pedal is high and hard, until I start the engine and now have a working booster. With the engine running, I can push the pedal to the floor with almost no resistance, almost like something was broken.
The booster has a sticker on it, saying checked and approved, it's a reman unit with no name tag to identify who did it.
I'm assuming the booster is screwed, and it looks like a fun job to replace.
Time for 2 or 7 beers.
I have bench bled the master cylinder on the car following the instructions posted here. The front half had no air, the back half had about 10 bubbles about the size you get in soda.
The pedal is high and hard, until I start the engine and now have a working booster. With the engine running, I can push the pedal to the floor with almost no resistance, almost like something was broken.
The booster has a sticker on it, saying checked and approved, it's a reman unit with no name tag to identify who did it.
I'm assuming the booster is screwed, and it looks like a fun job to replace.
Time for 2 or 7 beers.
Race Director
Joined: Nov 2006
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From: Sunshine Coast, Queensland, Australia
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It does sound like the booster is screwed. Have you checked it out closely? Tested it to see if it holds vacuum?
changing it out is a pain.
changing it out is a pain.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
This is not a hydraulic issue, so the brakes, lines, MC and calipers are fine. Congratulate yourself on a firm pedal and whack yourself on the back. You are 90% there on a brake-job.
Keep in mind the booster is strictly an assist. It really doesn't have anything to due with the brake system holding pressure, which yours does.
What is the history on the MC? Something you just bought and installed? Is it the correct unit? The reason I ask is, I would question the MCs gap between the MC piston and the booster rod. I don't believe a '76 has an adjustable booster rod, nor is there an optional hole in the brake pedal assembly under the dash. If that gap is not correct (0.060) then that distance is multiplied when the booster is activated.
Could be a faulty booster, but they seldom fail, just not much to it. You could always double check the vacuum port fitting for a perfect seal.
And if you have one of those vacuum hand pump tools, you could apply vacuum to the booster line at the "carb end" of the line to see what happens.
Keep in mind the booster is strictly an assist. It really doesn't have anything to due with the brake system holding pressure, which yours does.
What is the history on the MC? Something you just bought and installed? Is it the correct unit? The reason I ask is, I would question the MCs gap between the MC piston and the booster rod. I don't believe a '76 has an adjustable booster rod, nor is there an optional hole in the brake pedal assembly under the dash. If that gap is not correct (0.060) then that distance is multiplied when the booster is activated.
Could be a faulty booster, but they seldom fail, just not much to it. You could always double check the vacuum port fitting for a perfect seal.
And if you have one of those vacuum hand pump tools, you could apply vacuum to the booster line at the "carb end" of the line to see what happens.
Last edited by HeadsU.P.; Aug 23, 2020 at 08:01 AM.
Melting Slicks
Joined: Nov 1999
Posts: 2,281
Likes: 129
From: NOVA
St. Jude Donor '05-'06-'07
I have bled my '76 brakes by gravity, and with a Motive Power and there is no air in it at all.
I have bench bled the master cylinder on the car following the instructions posted here. The front half had no air, the back half had about 10 bubbles about the size you get in soda.
The pedal is high and hard, until I start the engine and now have a working booster. With the engine running, I can push the pedal to the floor with almost no resistance, almost like something was broken.
The booster has a sticker on it, saying checked and approved, it's a reman unit with no name tag to identify who did it.
I'm assuming the booster is screwed, and it looks like a fun job to replace.
Time for 2 or 7 beers.
I have bench bled the master cylinder on the car following the instructions posted here. The front half had no air, the back half had about 10 bubbles about the size you get in soda.
The pedal is high and hard, until I start the engine and now have a working booster. With the engine running, I can push the pedal to the floor with almost no resistance, almost like something was broken.
The booster has a sticker on it, saying checked and approved, it's a reman unit with no name tag to identify who did it.
I'm assuming the booster is screwed, and it looks like a fun job to replace.
Time for 2 or 7 beers.
Drifting
Joined: Nov 2016
Posts: 1,957
Likes: 576
From: Pittsburgh PA
I had a similar issue with my 68. Was the booster.
you can try to isolate the booster and see if it holds vacuum as a test.
my good booster I can pump and hold vacuum with a hand pump. Bad booster nada
you can try to isolate the booster and see if it holds vacuum as a test.
my good booster I can pump and hold vacuum with a hand pump. Bad booster nada
Racer
Joined: Oct 2018
Posts: 457
Likes: 132
From: England
I agree with MelWff. If the booster doesn't provide any assist then it will feel the same as with the engine off. My guess is that the booster IS providing assist so in effect you are pressing much harder on the brake pedal. If it then goes to the floor I'd say that a seal is letting by somewhere. If it is one in a caliper you'll have a leak, if there's no leak it could be a master cylinder seal.
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Thread Starter
Racer
Joined: Nov 2019
Posts: 336
Likes: 173
From: Canadian living in southern France
The play in the push rod has been adjusted and checked.
The master cylinder was on the car when I bought it, and worked OK.
When the engine isn't running, and no vacuum to the booster, the pedal is high and hard, and stays that way. I had a friend of mine push the pedal while I tried to spin a wheel, no go, he released the pedal and the wheel spun freely, as soon as he pushed the pedal, it locked right up. As soon as I start the engine and push the pedal, it goes to the floor. When I bench bled the master cylinder, the fluid gushed through the tubes back into the reservoir, so it's definitely pushing fluid.
For sure, these brakes are a little weird, but in 50 years, I've never seen anything like this.
I don't like "throwing parts" at a problem, especially here in France, where with shipping, taxes, duty etc, it costs me double.
I'm going to make sure the brakes will stop the car in front of my garage, (with the vacuum hose to the booster disconnected and plugged), and drive it carefully to my buddy's place, get it up on the hoist, and............I don't know what? Convert to manual brakes? Trade it for a Mustang?
This is starting to drive me nuts.
The master cylinder was on the car when I bought it, and worked OK.
When the engine isn't running, and no vacuum to the booster, the pedal is high and hard, and stays that way. I had a friend of mine push the pedal while I tried to spin a wheel, no go, he released the pedal and the wheel spun freely, as soon as he pushed the pedal, it locked right up. As soon as I start the engine and push the pedal, it goes to the floor. When I bench bled the master cylinder, the fluid gushed through the tubes back into the reservoir, so it's definitely pushing fluid.
For sure, these brakes are a little weird, but in 50 years, I've never seen anything like this.
I don't like "throwing parts" at a problem, especially here in France, where with shipping, taxes, duty etc, it costs me double.
I'm going to make sure the brakes will stop the car in front of my garage, (with the vacuum hose to the booster disconnected and plugged), and drive it carefully to my buddy's place, get it up on the hoist, and............I don't know what? Convert to manual brakes? Trade it for a Mustang?
This is starting to drive me nuts.
Melting Slicks
Joined: Jan 2004
Posts: 3,247
Likes: 210
From: Detroit MI
Does it really go to the floor? The pedal drop is normal when the engine starts. The boosters on these cars provide so much assist that they turn the pedal to mush, and it takes almost no foot pressure to stop the car. Did you do the spinning wheel test with the engine started?
Drifting
Joined: Nov 2016
Posts: 1,957
Likes: 576
From: Pittsburgh PA
do a vacuum test on the booster. on a good booster, I was able to use a hand pump and it would hold vacuum. it took quite a few pumps, but it would hold. on my bad booster, it would not hold vacuum.
does it still stop the car even though the pedal goes to the floor? mine would stop the car quickly, but I could just keep pushing it down.
here is a thread of mine where i had a similar issue.
https://www.corvetteforum.com/forums...in-brakes.html
does it still stop the car even though the pedal goes to the floor? mine would stop the car quickly, but I could just keep pushing it down.
here is a thread of mine where i had a similar issue.
https://www.corvetteforum.com/forums...in-brakes.html
Race Director
Joined: May 2006
Posts: 18,690
Likes: 2,572
The play in the push rod has been adjusted and checked.
The master cylinder was on the car when I bought it, and worked OK.
When the engine isn't running, and no vacuum to the booster, the pedal is high and hard, and stays that way. I had a friend of mine push the pedal while I tried to spin a wheel, no go, he released the pedal and the wheel spun freely, as soon as he pushed the pedal, it locked right up. As soon as I start the engine and push the pedal, it goes to the floor. When I bench bled the master cylinder, the fluid gushed through the tubes back into the reservoir, so it's definitely pushing fluid.
For sure, these brakes are a little weird, but in 50 years, I've never seen anything like this.
I don't like "throwing parts" at a problem, especially here in France, where with shipping, taxes, duty etc, it costs me double.
I'm going to make sure the brakes will stop the car in front of my garage, (with the vacuum hose to the booster disconnected and plugged), and drive it carefully to my buddy's place, get it up on the hoist, and............I don't know what? Convert to manual brakes? Trade it for a Mustang?
This is starting to drive me nuts.
The master cylinder was on the car when I bought it, and worked OK.
When the engine isn't running, and no vacuum to the booster, the pedal is high and hard, and stays that way. I had a friend of mine push the pedal while I tried to spin a wheel, no go, he released the pedal and the wheel spun freely, as soon as he pushed the pedal, it locked right up. As soon as I start the engine and push the pedal, it goes to the floor. When I bench bled the master cylinder, the fluid gushed through the tubes back into the reservoir, so it's definitely pushing fluid.
For sure, these brakes are a little weird, but in 50 years, I've never seen anything like this.
I don't like "throwing parts" at a problem, especially here in France, where with shipping, taxes, duty etc, it costs me double.
I'm going to make sure the brakes will stop the car in front of my garage, (with the vacuum hose to the booster disconnected and plugged), and drive it carefully to my buddy's place, get it up on the hoist, and............I don't know what? Convert to manual brakes? Trade it for a Mustang?
This is starting to drive me nuts.
Last edited by MelWff; Aug 24, 2020 at 12:13 PM.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
Any time I had that happen the brakes still had air in them somewhere.
Stopping a wheel from turning by hand takes hardly any brake pressure. Take it for a drive and see if it stops.
There should be no way for the pedal to go to the floor without also moving the master piston if the booster and master are properly installed. The booster just doesn't have that much play internally. The only part between the pedal rod and the master rod that can compress is the reaction disc. This is a rubber disk probably about 3/8*" thick. Once you compress it, the pedal rod must move the master rod. The compression of the reaction disc is what activates the internal valving to apply boost.
Stopping a wheel from turning by hand takes hardly any brake pressure. Take it for a drive and see if it stops.
There should be no way for the pedal to go to the floor without also moving the master piston if the booster and master are properly installed. The booster just doesn't have that much play internally. The only part between the pedal rod and the master rod that can compress is the reaction disc. This is a rubber disk probably about 3/8*" thick. Once you compress it, the pedal rod must move the master rod. The compression of the reaction disc is what activates the internal valving to apply boost.
Thread Starter
Racer
Joined: Nov 2019
Posts: 336
Likes: 173
From: Canadian living in southern France
Does it really go to the floor? The pedal drop is normal when the engine starts. The boosters on these cars provide so much assist that they turn the pedal to mush, and it takes almost no foot pressure to stop the car. Did you do the spinning wheel test with the engine started?
Thread Starter
Racer
Joined: Nov 2019
Posts: 336
Likes: 173
From: Canadian living in southern France
do a vacuum test on the booster. on a good booster, I was able to use a hand pump and it would hold vacuum. it took quite a few pumps, but it would hold. on my bad booster, it would not hold vacuum.
does it still stop the car even though the pedal goes to the floor? mine would stop the car quickly, but I could just keep pushing it down.
here is a thread of mine where i had a similar issue.
https://www.corvetteforum.com/forums...in-brakes.html
does it still stop the car even though the pedal goes to the floor? mine would stop the car quickly, but I could just keep pushing it down.
here is a thread of mine where i had a similar issue.
https://www.corvetteforum.com/forums...in-brakes.html
Thread Starter
Racer
Joined: Nov 2019
Posts: 336
Likes: 173
From: Canadian living in southern France
No, not for that long, and I see your point. I drove it to my friend's place, about 5 minutes, and by the time I got there, there was almost no brakes left, even with the booster unhooked.
Thread Starter
Racer
Joined: Nov 2019
Posts: 336
Likes: 173
From: Canadian living in southern France
Any time I had that happen the brakes still had air in them somewhere.
Stopping a wheel from turning by hand takes hardly any brake pressure. Take it for a drive and see if it stops.
There should be no way for the pedal to go to the floor without also moving the master piston if the booster and master are properly installed. The booster just doesn't have that much play internally. The only part between the pedal rod and the master rod that can compress is the reaction disc. This is a rubber disk probably about 3/8*" thick. Once you compress it, the pedal rod must move the master rod. The compression of the reaction disc is what activates the internal valving to apply boost.
Stopping a wheel from turning by hand takes hardly any brake pressure. Take it for a drive and see if it stops.
There should be no way for the pedal to go to the floor without also moving the master piston if the booster and master are properly installed. The booster just doesn't have that much play internally. The only part between the pedal rod and the master rod that can compress is the reaction disc. This is a rubber disk probably about 3/8*" thick. Once you compress it, the pedal rod must move the master rod. The compression of the reaction disc is what activates the internal valving to apply boost.
I'm thinking, (hoping), that when I'm done swearing after replacing and bleeding the M/C and booster, my problems will be over.
When this is figured out, I will post what I found, hopefully to help anybody else who has torn out the same amount of hair I have.