1980 Brake Booster Issues
Thread Starter
Heel & Toe
Joined: Jan 2021
Posts: 15
Likes: 1
From: Kentucky
1980 Brake Booster Issues
Hey all, I’m having a bit of trouble with the brakes on my ‘80. I’ve replaced the brake booster and master cylinder as well as almost every brake line on the car and I’m ready to get it back on the road, yet issues keep popping up. I’ve fully bled the master cylinder and the rest of the brake system and I haven’t got a single leak anywhere, I didn’t really have any problems there. When the car is off, I have a rock solid brake pedal. However, when the car is running the pedal sinks straight to the floor and the engine revs up a little. I know this to commonly be a bad brake booster, so I figured it was defective. I had it under warranty so I got a replacement in and installed it, which yielded the same result, pedal to the floor when I ran the car with an increase in RPM. At this point I’m pretty stumped. I’ve got no vacuum or fluid leaks anywhere and I’d assume that there’s a pretty low chance I’d get two defective boosters in a row. Does anybody know of something I could be missing here?
Drifting
Joined: Aug 2019
Posts: 1,509
Likes: 498
Are you sure there is a problem ? The reason you bleed brakes with engine off is so you can “feel “ the brake pedal that you lose the feel with the engine running and vacuum booster working.
If you put it in gear while running does the car stop immediately at the touch of the pedal or does it go to the floor and then stop ? Or not stop at all ?
If you put it in gear while running does the car stop immediately at the touch of the pedal or does it go to the floor and then stop ? Or not stop at all ?
Thread Starter
Heel & Toe
Joined: Jan 2021
Posts: 15
Likes: 1
From: Kentucky
Are you sure there is a problem ? The reason you bleed brakes with engine off is so you can “feel “ the brake pedal that you lose the feel with the engine running and vacuum booster working.
If you put it in gear while running does the car stop immediately at the touch of the pedal or does it go to the floor and then stop ? Or not stop at all ?
If you put it in gear while running does the car stop immediately at the touch of the pedal or does it go to the floor and then stop ? Or not stop at all ?
Racer
Joined: Oct 2018
Posts: 457
Likes: 132
From: England
I can't see how a defective brake booster would give that problem. The booster allows you to apply more force to the master cylinder, so if you press the brake pedal a few times with the engine off (to get rid of any vacuum in the booster) then start the engine with your foot on the brake the pedal should go down. If the booster isn't working the brake pedal won't move down any further. I'd say you either still have some air in the system that you can't feel without the power assistance, or your master cylinder seals are letting by. Did you bench bleed the master cylinder? If so, did you overstroke it, I'm told this can damage the seals.
Thread Starter
Heel & Toe
Joined: Jan 2021
Posts: 15
Likes: 1
From: Kentucky
I can't see how a defective brake booster would give that problem. The booster allows you to apply more force to the master cylinder, so if you press the brake pedal a few times with the engine off (to get rid of any vacuum in the booster) then start the engine with your foot on the brake the pedal should go down. If the booster isn't working the brake pedal won't move down any further. I'd say you either still have some air in the system that you can't feel without the power assistance, or your master cylinder seals are letting by. Did you bench bleed the master cylinder? If so, did you overstroke it, I'm told this can damage the seals.
Racer
Joined: Oct 2018
Posts: 457
Likes: 132
From: England
Yes that's what I mean. I've seen several threads here that say you musn't push the piston all the way in. My guess is that you need new seals for the master cylinder, but I don't have first hand experience of this problem. Hopefully someone else on here who has actually had this problem can confirm that this is the likely issue.
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Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Never shove your phillips screwdriver / MC piston in more than 1 & 3/8 inches. There should have been a warning label on the box. This action will void warranty.
When bench bleeding, you are forcing the piston in farther than a brake pedal would normally do. This can and will make the piston seals catch on the inner casting and rolls the seal over, damaging them.
Click on my profile, then photo albums > bench bleed.
When bench bleeding, you are forcing the piston in farther than a brake pedal would normally do. This can and will make the piston seals catch on the inner casting and rolls the seal over, damaging them.
Click on my profile, then photo albums > bench bleed.
Last edited by HeadsU.P.; Mar 4, 2021 at 04:04 PM.
Thread Starter
Heel & Toe
Joined: Jan 2021
Posts: 15
Likes: 1
From: Kentucky
Never shove your phillips screwdriver / MC piston in more than 1 & 3/8 inches. There should have been a warning label on the box. This action will void warranty.
When bench bleeding, you are forcing the piston in farther than a brake pedal would normally do. This can and will make the piston seals catch on the inner casting and rolls the seal over, damaging them.
Click on my profile, then photo albums > bench bleed.
When bench bleeding, you are forcing the piston in farther than a brake pedal would normally do. This can and will make the piston seals catch on the inner casting and rolls the seal over, damaging them.
Click on my profile, then photo albums > bench bleed.
Race Director
Joined: Jul 2018
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From: San Antonio, TX/Mahopac, NY
2024 Corvette of the Year Finalist - Modified
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You have a bad MC. Your symptoms indicate the hose and booster are fine. Rebuild the new one and you should be good to go.
Racer
Joined: Mar 2015
Posts: 306
Likes: 20
From: Greenville SC
I hate to be the bearer of bad news, however, I have a 79 and have had the same issue as you. Started last year about this time. It's still on jack stands. 3 boosters, 3 M/C , New calipers, new hoses, New prop. valve. Over $ 1300.00 still no brakes. Did I mention 5 different mechanics. Bled 1000 times gravity, vacuum, pressure. Check out my last two posts. I hate this car.
Safety Car
Joined: Jul 2005
Posts: 3,694
Likes: 833
From: WI
Nobody mentioned the booster Check Valve (for the OP). The engine shouldn't rev as ya push the brakes down. A bad check valve could cause that. Remove the check valve and blow on it. Not that expensive to replace (Rock Auto has them cheap). Pedal going all the way to the floor is probably the MC as others have said (or not fully bled out).
Last edited by Mark G; Mar 7, 2021 at 10:44 PM.
Thread Starter
Heel & Toe
Joined: Jan 2021
Posts: 15
Likes: 1
From: Kentucky
Well I’m completely stumped here. We bled and replaced my master cylinder with a brand new one and bled all the brakes, got the same result. We checked the check valve and the booster is holding vacuum just fine. I’ve done everything I can think of with no result. Ive got no idea what’s going on here.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
You need to isolate the MC from the rest of the system. You have to find out which part is the culprit here.
Go to your favorite automotive store. Look for ready made (pre fitted) brake lines about a foot long. They will have the brass fittings on the end.
Note: As you know, the front & rear MC line ports are different diameter and different thread pitch. Off the top of my head, I don't remember the size.
Buy one of each size front & rear fitting line. This will be your new "MC Test Lines". You will need to block off the other end with fittings, caps or bend / pinch it.
Remove your old front & rear brake lines at the MC. Install your test lines. Bend them upwards to clear fender. If the MC is perfect, you will be able to stand on the pedal w/o any movement. If you have a solid pedal, look elsewhere.
You will need to do a four corner bleed again after the test. You introduced air.
Go to your favorite automotive store. Look for ready made (pre fitted) brake lines about a foot long. They will have the brass fittings on the end.
Note: As you know, the front & rear MC line ports are different diameter and different thread pitch. Off the top of my head, I don't remember the size.
Buy one of each size front & rear fitting line. This will be your new "MC Test Lines". You will need to block off the other end with fittings, caps or bend / pinch it.
Remove your old front & rear brake lines at the MC. Install your test lines. Bend them upwards to clear fender. If the MC is perfect, you will be able to stand on the pedal w/o any movement. If you have a solid pedal, look elsewhere.
You will need to do a four corner bleed again after the test. You introduced air.
Last edited by HeadsU.P.; Mar 8, 2021 at 08:02 PM.