Soft brake pedal
Thread Starter
Heel & Toe
Joined: Nov 2020
Posts: 17
Likes: 2
Soft brake pedal
Help Please.... On my 81 vette I had brake pedal that went to the floor. Bleeding the brakes didn't help. I replaced all 4 calipers and a new master cylinder. Still had no brake pedal.The brakes have been bled a lot, I even bought the power bleeder. SO..when I take the hose "off" the brake booster (the cars rpm goes up a little, BUT when I push on the brakes I have a hard pedal. " Does this mean the brake booster is bad??" There is good vacume coming from the engine hose. Do I need a brake booster or is it from something else..Could the vacuum hose filter or vacuum check valve be bad? It seems like a vacuumed thing because when I disconnect the power booster . Then I have a harder pedal. Thank you.
So thankful there is so much experienced people here. You have helped me when I had a wire not making contact.
So thankful there is so much experienced people here. You have helped me when I had a wire not making contact.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
First question.
Hundreds more to follow.
Was the MC "bench bled"?
Do not confuse a booster system with the entire brake system.
One is a vacuum assist system, one is a hydraulic system.
Your RPMs increased because you introduced a vacuum leak. Boosters seldom go bad. But those one-way check valves do.
A hard pedal should happen regardless if a booster is good / bad or even if a booster is not used. You do not have a consistent pedal. So, I suspect there is still air somewhere in the system.
Hundreds more to follow.
Was the MC "bench bled"?
Do not confuse a booster system with the entire brake system.
One is a vacuum assist system, one is a hydraulic system.
Your RPMs increased because you introduced a vacuum leak. Boosters seldom go bad. But those one-way check valves do.
A hard pedal should happen regardless if a booster is good / bad or even if a booster is not used. You do not have a consistent pedal. So, I suspect there is still air somewhere in the system.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
That I don't know.
But keep in mind the booster is strictly an assist so you don't have to push the brake pedal so hard.
You have a hydraulic brake pedal to the floor. The vacuum booster and check valve can wait.
After you bench bled and hooked the two brake lines up, did the entire system get bled? When those line are attached to the MC, it is impossible to not a have a bubble there at that location.
I take it all six bleeders are being opened in the proper sequence?
But keep in mind the booster is strictly an assist so you don't have to push the brake pedal so hard.
You have a hydraulic brake pedal to the floor. The vacuum booster and check valve can wait.
After you bench bled and hooked the two brake lines up, did the entire system get bled? When those line are attached to the MC, it is impossible to not a have a bubble there at that location.
I take it all six bleeders are being opened in the proper sequence?
Burning Brakes
Joined: Apr 2020
Posts: 788
Likes: 418
From: Western NY
Sometimes after you bench bleed the MC and install it if you attach the lines and tighten the fittings then have someone depress the pedal while you crack the lines open you can push some if not all the air out where the lines join the MC. HOWEVER make sure you wrap a towel around the fittings so brake fluid doesn't spray everywhere. obviously this will do nothing for any air that is downstream of the MC.
Thread Starter
Heel & Toe
Joined: Nov 2020
Posts: 17
Likes: 2
I bench bled the MC, hooked the lines up to it and proceed to bleed all 4 calipers( I cant tell u how many times I have gone around that car..lol) I bled according to what the shop manual says..LR-inner_ LR-outer...RR-inner RR Outer..Front LT and Front-right. I have been bleeding the brakes off and on all day Its FL and HOT..I don't have a garage..lol I don't know what else to do but just keep going around the car in sequence.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
When you bench bled the MC, you didn't force the piston too far in its bore did you? That will damage the seals & void any warranty = pedal to the floor.
Take a peek at my profile > photo album > bench bleeding for some tips.
Take a peek at my profile > photo album > bench bleeding for some tips.
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Thread Starter
Heel & Toe
Joined: Nov 2020
Posts: 17
Likes: 2
Oh no...I dont think I did (bench bleed piston too far)....Is there any way to tell if I have? Now I'm wondering if my old one was bad. Perhaps I should put it back on and start from scratch? Or buy another one? Thank you so much for helping me.
Last edited by RatFink60racing; Jun 9, 2021 at 07:04 PM. Reason: Add Text
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
The only true way to test the MC for a pressure build up test is to isolate it from the brake system. This test is performed with the MC mounted on the car.
The pedal will easily check pressure, something that can never be achieved in a vise on the bench.
Basically you are seeing if the MC will hold pressure. The front & rear brake line ports will have to be blocked off. Do not use bolts, you will damaged the flare inside.
Both lines will be disconnected, the MC should be full w/ DOT3.
You will need to fabricate two test lines. NAPA will have short lengths of premade brake lines with fittings. Note that each port on the MC is different diameter as well as different TPI. Take that info with you to NAPA. I don't recall the sizes.
Block off the other end of the test lines by pinching off the tubing or block it using some brass fittings, and get creative.
Install the two test lines. You should now be able to stand on the brake pedal, all day long.
If the pedal sinks, the MC seals are shot.
The pedal will easily check pressure, something that can never be achieved in a vise on the bench.
Basically you are seeing if the MC will hold pressure. The front & rear brake line ports will have to be blocked off. Do not use bolts, you will damaged the flare inside.
Both lines will be disconnected, the MC should be full w/ DOT3.
You will need to fabricate two test lines. NAPA will have short lengths of premade brake lines with fittings. Note that each port on the MC is different diameter as well as different TPI. Take that info with you to NAPA. I don't recall the sizes.
Block off the other end of the test lines by pinching off the tubing or block it using some brass fittings, and get creative.
Install the two test lines. You should now be able to stand on the brake pedal, all day long.
If the pedal sinks, the MC seals are shot.
Last edited by HeadsU.P.; Jun 9, 2021 at 08:35 PM.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
The first MC I bought do not clearly warn me about over-extension of the piston. The second MC purchased had a big, yellow warning label on the box.
There are repair kits for rebuilding an MC. The biggest drawback of a repair is, if the bore of the MC is pitted / rusty and in need of a honing. That would not be the case of a somewhat fresh damaged unit.
I hope you ordered the correct MC. That's another simple task that could go wrong and often does according to this forum.
There are repair kits for rebuilding an MC. The biggest drawback of a repair is, if the bore of the MC is pitted / rusty and in need of a honing. That would not be the case of a somewhat fresh damaged unit.
I hope you ordered the correct MC. That's another simple task that could go wrong and often does according to this forum.
Racer
Joined: Nov 2019
Posts: 336
Likes: 173
From: Canadian living in southern France
I went through the same situation not long ago. Without doubt, you have air in the system. It is extremely hard to get it all out. It took me quite a number of tries, even with a pressure bleeder.
I finally ended up with a so-so brake pedal which got me through an inspection for the title transfer.
But, the only way I found to get rid of all the air was to bleed the brakes backwards. I will post a description and a couple of pictures tomorrow. Everybody will think I'm crazy, but it worked.
Stay tuned.
I know it seems hopeless, it sure did to me, but you will get there, and believe me, it's worth it, I've never, in fifty years, had a car that was more fun to drive.
I finally ended up with a so-so brake pedal which got me through an inspection for the title transfer.
But, the only way I found to get rid of all the air was to bleed the brakes backwards. I will post a description and a couple of pictures tomorrow. Everybody will think I'm crazy, but it worked.
Stay tuned.
I know it seems hopeless, it sure did to me, but you will get there, and believe me, it's worth it, I've never, in fifty years, had a car that was more fun to drive.
Le Mans Master
Joined: Jun 2019
Posts: 5,431
Likes: 2,155
From: Loveland
Pushing a new MC piston in all the way will NOT damage the seals. There is nothing in the cylinder to catch on the seals. There is also a rod on the end of the piston to limit how far you can push it in!!!!! This is an OLD WIVES TALE. Take a MC apart and see for yourself. I have had 3 of them apart, all the same. Push away.
Last edited by kodpkd; Jun 12, 2021 at 11:45 AM.
Le Mans Master
Joined: Jun 2019
Posts: 5,431
Likes: 2,155
From: Loveland
The only way I can see this happening, is if your MC has a so much rust inside the bore, that the seal catches on the rust ridge. If there is that much rust inside the bore maybe it should be replaced. OR if the MC has a small bolt inside the bore that stops the piston from moving to far, I haven't seen this in a C3 MC.
