Soft brakes
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
Soft brakes
I replaced the brake booster about a year ago and ever since then the brakes have been soft with alot of travel.... the brakes still bite hard and will brake well but the pedal feels soft and spongy... I just flushed the brake fluid and it helped a little but I still don't like how the pedal feels. It does not add confidence when your driving as I am used to a firmer brake pedal. Any suggestions what can be wrong?
Melting Slicks
Joined: May 2019
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From: Mid-Atlantic
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What I found helpful and to result in an improvement was to work the ABS on a good gravel road.
Don't have to go fast. Just work it a few times. Though I worked em for probably ten minutes.
Then go home and bleed again.
I'd read that on here some time ago, so tried it and it did improve the brakes.
Guess the booster pushrod could always be in need of a minor adjustment, too. Dunno...
Don't have to go fast. Just work it a few times. Though I worked em for probably ten minutes.
Then go home and bleed again.
I'd read that on here some time ago, so tried it and it did improve the brakes.
Guess the booster pushrod could always be in need of a minor adjustment, too. Dunno...
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
What I found helpful and to result in an improvement was to work the ABS on a good gravel road.
Don't have to go fast. Just work it a few times. Though I worked em for probably ten minutes.
Then go home and bleed again.
I'd read that on here some time ago, so tried it and it did improve the brakes.
Guess the booster pushrod could always be in need of a minor adjustment, too. Dunno...
Don't have to go fast. Just work it a few times. Though I worked em for probably ten minutes.
Then go home and bleed again.
I'd read that on here some time ago, so tried it and it did improve the brakes.
Guess the booster pushrod could always be in need of a minor adjustment, too. Dunno...
Melting Slicks
Joined: May 2019
Posts: 3,204
Likes: 1,387
From: Mid-Atlantic
2025 C4 of the Year Finalist - Unmodified
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But..I don't think that adjustment is relevant to ''spongy'' brakes. That adjustment is more for pedal distance before the brakes/MC engage.
And that adjustment, if I recall, has to be done before you do any other brake work...or...youhave to bleed the caliper first and then do the adjustment on that. I forget the right sequence. There's somebody around here who knows more than I do about it, gare awn teed.
Here's a good explanation, though, of that specific adjustment...
Honestly, if you flushed em, I'd just work the ABS on a good gravel road for a bit and then go home and bleed em again.
Best to leave the brake booster stuff alone, in hind sight, as it isn't really relevant to the spongy brakes anyway, so far as I know. I probably shouldn't even have mentioned it. Just ads needless confusion.
Last edited by Natty C; May 26, 2025 at 12:16 AM.
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
There's a tool that you need to get the right measurement between the MC and the booster pushrod.
But..I don't think that adjustment is relevant to ''spongy'' brakes. That adjustment is more for pedal distance before the brakes/MC engage.
And that adjustment, if I recall, has to be done before you do any other brake work...or...youhave to bleed the caliper first and then do the adjustment on that. I forget the right sequence. There's somebody around here who knows more than I do about it, gare awn teed.
Here's a good explanation, though, of that specific adjustment...
https://youtu.be/YDlz7-X3S2g
Honestly, if you flushed em, I'd just work the ABS on a good gravel road for a bit and then go home and bleed em again.
Best to leave the brake booster stuff alone, in hind sight, as it isn't really relevant to the spongy brakes anyway, so far as I know. I probably shouldn't even have mentioned it. Just ads needless confusion.
But..I don't think that adjustment is relevant to ''spongy'' brakes. That adjustment is more for pedal distance before the brakes/MC engage.
And that adjustment, if I recall, has to be done before you do any other brake work...or...youhave to bleed the caliper first and then do the adjustment on that. I forget the right sequence. There's somebody around here who knows more than I do about it, gare awn teed.
Here's a good explanation, though, of that specific adjustment...
https://youtu.be/YDlz7-X3S2g
Honestly, if you flushed em, I'd just work the ABS on a good gravel road for a bit and then go home and bleed em again.
Best to leave the brake booster stuff alone, in hind sight, as it isn't really relevant to the spongy brakes anyway, so far as I know. I probably shouldn't even have mentioned it. Just ads needless confusion.
Racer
Joined: Feb 2024
Posts: 267
Likes: 113
From: New York
Yes. I would adjust the brake booster pushrod. Adjust it about 1/2 turn at a time and retest until you have limited freeplay in the pedal. You'll get much better brake feel when it's adjusted properly. And remember that you must maintain a little bit of freeplay or the brakes will start to drag.
After you've removed the 2 bolts securing the master cylinder, it's helpful to have someone sit in the car and slowly push the brake pedal down part way until you can access the pushrod. Hold the knurled part with a small vise grip pliers to keep it from turning while you rotate the hex end with a wrench.
After you've removed the 2 bolts securing the master cylinder, it's helpful to have someone sit in the car and slowly push the brake pedal down part way until you can access the pushrod. Hold the knurled part with a small vise grip pliers to keep it from turning while you rotate the hex end with a wrench.
Melting Slicks
Joined: Oct 2003
Posts: 2,171
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From: Chapel Hill NC
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I would adjust the brake booster rod as recommended but it is my understanding that a soft and spongy brake pedal usually means air in the brake system.
Racer
Joined: Feb 2024
Posts: 267
Likes: 113
From: New York
I've made this adjustment on a few C4s and I've found that the long pedal travel actually makes the brakes "seem" spongy even if there's no air in the system.
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Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
Racer
Joined: Feb 2024
Posts: 267
Likes: 113
From: New York
Here's another test for pushrod length:
With the engine off, jack up one of the front wheels. Then, have an assistant rotate that wheel by hand while you slowly push on the brake pedal. Have your assistant tell you when they start to feel resistance from the brake pads. That pedal movement is your freeplay.
Team Owner
Joined: Jan 2003
Posts: 20,305
Likes: 240
From: Summerland B.C. Canada
I replaced my booster last year and I too just threw it in and would dial it in better at another time.
Today was that time as I noticed my master started leaking down the front of booster.
I don’t have the fancy gauge, so this is what I did.
I loosened pushrod adjustment nut and wound it out until it caused a gap between master and booster.
I then wound it back in a bit and added a drop of latex paint on the end of pushrod.
Wound it out a little at a time and slid on master, until the paint would leave a small trace on master piston.
I googled the gap and Ai said 20 thousand was good.
My blob of paint was 25 thousand by eye, so I called it good.
Tomorrow I have to bleed all 4 wheels, so I can’t comment on outcome yet 😆
Today was that time as I noticed my master started leaking down the front of booster.
I don’t have the fancy gauge, so this is what I did.
I loosened pushrod adjustment nut and wound it out until it caused a gap between master and booster.
I then wound it back in a bit and added a drop of latex paint on the end of pushrod.
Wound it out a little at a time and slid on master, until the paint would leave a small trace on master piston.
I googled the gap and Ai said 20 thousand was good.
My blob of paint was 25 thousand by eye, so I called it good.
Tomorrow I have to bleed all 4 wheels, so I can’t comment on outcome yet 😆
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
I replaced my booster last year and I too just threw it in and would dial it in better at another time.
Today was that time as I noticed my master started leaking down the front of booster.
I don’t have the fancy gauge, so this is what I did.
I loosened pushrod adjustment nut and wound it out until it caused a gap between master and booster.
I then wound it back in a bit and added a drop of latex paint on the end of pushrod.
Wound it out a little at a time and slid on master, until the paint would leave a small trace on master piston.
I googled the gap and Ai said 20 thousand was good.
My blob of paint was 25 thousand by eye, so I called it good.
Tomorrow I have to bleed all 4 wheels, so I can’t comment on outcome yet 😆
Today was that time as I noticed my master started leaking down the front of booster.
I don’t have the fancy gauge, so this is what I did.
I loosened pushrod adjustment nut and wound it out until it caused a gap between master and booster.
I then wound it back in a bit and added a drop of latex paint on the end of pushrod.
Wound it out a little at a time and slid on master, until the paint would leave a small trace on master piston.
I googled the gap and Ai said 20 thousand was good.
My blob of paint was 25 thousand by eye, so I called it good.
Tomorrow I have to bleed all 4 wheels, so I can’t comment on outcome yet 😆
Team Owner
Joined: Jan 2003
Posts: 20,305
Likes: 240
From: Summerland B.C. Canada
I don’t see the gap info in manual, just them showing factory parts and using a factory gauge.
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
Update.... I adjusted the gap which really didn't seem off but the pedal still feels soft with too much travel... I don't know what else I can do as there does not appear to be any air in the system. What can I do to firm up the pedal and make it feel more responsive throughout the pedal stroke?
Melting Slicks
Joined: Oct 2003
Posts: 2,171
Likes: 820
From: Chapel Hill NC
2025 c3 ('68-'73) of the Year Finalist - Unmodified
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Rereading this thread I don't see any mention of when the rubber flexible brake lines were changed. Old lines could possibly be giving the brakes a spongy feel. Many feel if there're 10 years or older they should be changed regardless.
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
the rubber brake lines that go from the hard lines to all of the calipers have all been replaced with braided steel lines about 10 years ago.