panic stop, hard pedal, weak braking

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snoddy
Posts: 7
Joined: Wed Oct 11, 2023 2:22 pm
Location: Champaign, IL

panic stop, hard pedal, weak braking

Post by snoddy »

This past weekend, I had to stab the brake pedal to slow down quickly. The pedal barely moved down despite my hard, quick push. The car did not slow down but the pedal felt very hard even though it was only in the top inch of travel. The brake pedal pushed past this "hard spot" a second later and I was able to stop.

Under normal driving, the pedal feels fine and braking performance is normal.
Pads and rotors are in great shape.

This is the second time this happened. The first was before I replaced the clutch master and slave cylinder. After I replaced those parts, I bled the system, but not the entire brake system.

Searching this topic landed on a few things (some ideas from other BMW models too)
1. replace all brake fluid for the entire system and bleed entire system to ensure no air. That was my first thought too, but in the past, air in the brake lines lead to a soft pedal and did not exhibit the behavior I mentioned above.
2. check vacuum lines to the booster.

Any other ideas?
plip1953
Posts: 413
Joined: Mon Aug 13, 2018 4:15 pm
Location: Leeds

Re: panic stop, hard pedal, weak braking

Post by plip1953 »

To have such a hard pedal to start with, and then for it to release, might possibly imply a stuck or sticky caliper piston. Exercising all the caliper pistons for free movement would probably be a sensible move. A less likely possibility might be a pad that is binding in it caliper housing.
Phil
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1988 Highline 635 CSi
snoddy
Posts: 7
Joined: Wed Oct 11, 2023 2:22 pm
Location: Champaign, IL

Re: panic stop, hard pedal, weak braking

Post by snoddy »

I ordered a a caliper repair kit that should get here next week so that when I take the calipers off again to inspect them, I can go ahead and replace the seals and inspect the pistons.

In the meantime, I took the car out tonight to test a few things. No matter what, if I jab the brake pedal, it barely moves down and is rock hard for a second before releasing and subsequently slowing the car. Everytime.

If I squeeze the pedal (rather than jab) and increase pressure on it rapidly, the car stops quickly and evenly.

It's only when you have to make a quick jab at the brake, like in a quick reaction situation, that the brakes don't work right away.

Very odd.
plip1953
Posts: 413
Joined: Mon Aug 13, 2018 4:15 pm
Location: Leeds

Re: panic stop, hard pedal, weak braking

Post by plip1953 »

I'm somewhat clutching at straws, but could another possibility be stickiness in the way the master cylinder pistons are moving? What you describe, in particular the way the pedal initially offers resistance and then releases (generally the story is the other way round!), doesn't fit naturally with anything I've come across before.
Phil
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1988 Highline 635 CSi
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Steve M6
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Joined: Sat Oct 07, 2006 8:51 pm
Location: Monterey, CA

Re: panic stop, hard pedal, weak braking

Post by Steve M6 »

Your profile and post does not give us any clue about your particular car.

I suspect that the "brake bomb", the nitrogen filled pressure ball underneath the master cylinder, is your problem. Very unlikely that the symptoms you are describing would be caused by the caliper pistons.

Let us know what you eventually find to fix the problem.
'86 635CSi 5-sp
'88 M6 Canadian
'03 530i 5-sp - ordered new, now gone, victim of someone else's spin-out on I880
'02 530i 5-sp - now my daily driver - finally replaced my original 530i
'96 Ford F-150, 5-sp too
snoddy
Posts: 7
Joined: Wed Oct 11, 2023 2:22 pm
Location: Champaign, IL

Re: panic stop, hard pedal, weak braking

Post by snoddy »

Ah! Sorry about that.
1984 633 csi.

I agree on the master cylinder. I have ordered a new one and will replace as soon as it gets here.
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Bimmermaven
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Joined: Tue May 07, 2024 8:28 pm
Location: USA-NC

Re: panic stop, hard pedal, weak braking

Post by Bimmermaven »

I'm thinking with Steve M6....a lack of boost.
if you're not familiar, disconnect vac booster hose and plug it on another car, then take a test drive to to calibrate your brain.

so, whether vac or hydraulic boost, the actuator valve that engages the boost device is unhappy. my 72 Bavaria did this when the booster was 8 years old. I had to replace the entire vac booster.

stuck wheel pistons would have to all 4 stick at once.

master cyl pistons would both need to stick at once.

either way, light pressure wouldn't loosen them.
kronus
Posts: 124
Joined: Fri Jun 18, 2021 2:20 am
Location: SF

Re: panic stop, hard pedal, weak braking

Post by kronus »

I have a similar issue sometimes (pedal gets very hard and brakes drag + weak braking). This thread has a lot of useful discussion about it - viewtopic.php?t=29004

tl;dr replace or rebuild hydroboost and MC.
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