Brake pedal randomly goes to the floor

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OnTheFritz
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Brake pedal randomly goes to the floor

Post by OnTheFritz »

Hi all, I've got quite the odd problem now. Over the last few months I've noticed some very rare and strange behavior with my brakes that I can't begin to diagnose or find other people with similar issues. The issue being very concerning brake pedal behavior, where out of the blue after working perfectly normal, the brake pedal will seemingly disconnect itself from the rest of the system. This has only happened twice out of the maybe 1000 miles the car's been driven over the last few months, never before now. Both times this has happened, I had recently come to a stop after some moderately spirited driving, not putting significant heat through the brakes. The brakes worked completely fine and as expected to bring the car to a stop, but after releasing and rolling along, trying to stop again from idle speeds, the pedal goes straight to the floor with no modulation until the last few millimeters of travel, at which point the brakes actually will grab though only in a binary on/off fashion. Shutting the car off and restarting does nothing to reset the issue, but both times the brakes have come back after 10 minutes or so to just as they were before the issue. It's as if some component under the hood starts to heat soak and starts acting all wonky (like my ICV now, which coincidentally started acting up around the same time...)

The brakes were fully flushed and bled in September of last year, and I have had zero braking troubles in the 10k miles since then until now. When the pedal has gone completely limp and "disconnected", there has been zero resistance in pushing it to the floor; it's as though there is suddenly a massive gap between the brake lever and master cylinder piston. Also while the pedal is limp, I tried building pressure in the brake bomb with the engine off, and after maybe 6-8 "pumps" of the pedal, ATF rose to the top and started spewing over, as a healthy system would do.

I'm completely stumped as to what the root issue could be. There have been no ABS warnings on the dash (the bulb illuminates in key position 2 like all others, so it's not burnt out), but I haven't had the proper circumstances to safely test ABS function. No puddles other than residue from spillage out of the brake bomb caused by myself. No traces of fluid leaks from any fittings or hoses throughout the braking system. I'm leaning towards a failing ABS unit at this point, but would love some second opinions and more experienced minds to bounce ideas off of.
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daz635
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Re: Brake pedal randomly goes to the floor

Post by daz635 »

Sounds like it could be master cylinder to me. A bad brake bomb would show up with different symptoms

Did you notice if one of the calipers was binding slightly when driven?

If one of the calipers is dragging slightly due to a sticking master cylinder the fluid will heat up in that circuit and can release into the system usually when bringing the car to a stop causing a super soft brake pedal

It will normally rectify itself after 10 minutes after the MC cools down. To test when it next occurs try poring some cold water over the master cylinder and you should hear it release

Have a read thru this thread viewtopic.php?f=17&t=16351&hilit=master+cylinder

Also FYI when pumping the brake pedal 20 times or until the pedal goes hard the atf fluid should be 10mm from the top of the power steering reservoir not overflowing.
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plip1953
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Re: Brake pedal randomly goes to the floor

Post by plip1953 »

daz635 wrote: Tue Aug 04, 2020 12:48 am

If one of the calipers is dragging slightly due to a sticking master cylinder the fluid will heat up in that circuit and can release into the system usually when bringing the car to a stop causing a super soft brake pedal
If the master cylinder piston gets itself stuck towards the end of its normal travel, wouldn't the brake pedal fail to release back up to it's normal resting position?

But what if master cylinder piston seal had failed? Wouldn't that allow the pedal to move pretty much freely through it's range of travel, but not actually produce braking pressure? But if the seal is failing/has failed, I wonder why the event only happens occasionally? Maybe heat from the exhaust manifold?

But overall I do tend to agree that the master cylinder is the most likely culprit.
Phil
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Re: Brake pedal randomly goes to the floor

Post by 86_6series »

+1 on the MC.
+1 on the PS fluid level.

Your pump is overfilled. Remove ATF
as you pump the brakes until it stops rising.
As noted the level should then be about 10mm from top.

In the future when checking level, without engine running-pump brake pedal until hard.
The bomb will then evacuate the fluid raising the level.
The level is measured with this procedure.
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OnTheFritz
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Re: Brake pedal randomly goes to the floor

Post by OnTheFritz »

So regarding slightly sticky calipers, I feel like I've heard/otherwise noticed some veerrry slight brake dragging over the last couple years, but I just thought it was the piston seals on the calipers (which are quite worn and in need of a refresh). I'm surprised I only get this issue after the engine bay heatsoaks, and in such a binary fashion, but I can believe it. Regarding the master cylinder troubleshooting guide, it seems like my car has some of the listed symptoms in one degree or another, so at this point I'm sold on the MC. This will give me a great excuse to upgrade to the E32 750 master cylinder like I've been thinking about for a while now :lol:

And as to the fluid fill level, I do understand I've been filling it a bit too full on a few occasions. When I first got the car and knew nothing, the previous owner told me "fill it up, let any excess leak out, repeat, and eventually you'll find it's happy level". For the longest time I thought it was being under-filled, but I suppose the car has spoken. The fluid level check outs well now, right where it should be at slightly below the rim when depressurized.
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Rondel Fan
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Re: Brake pedal randomly goes to the floor

Post by Rondel Fan »

The pedal moving through it's range but not producing braking power. Isn't this exactly what the pedal going to the floor is?
I'm firmly in the MC camp. If not changed yet, bring along a gallon container of water for future rides. If it happens again pour the water on the MC to cool it off and see if the pedal comes back. If so, you'll have both your answer, and temporary brakes..

[/quote]

If the master cylinder piston gets itself stuck towards the end of its normal travel, wouldn't the brake pedal fail to release back up to it's normal resting position?

But what if master cylinder piston seal had failed? Wouldn't that allow the pedal to move pretty much freely through it's range of travel, but not actually produce braking pressure? But if the seal is failing/has failed, I wonder why the event only happens occasionally? Maybe heat from the exhaust manifold?

But overall I do tend to agree that the master cylinder is the most likely culprit.
[/quote]
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Re: Brake pedal randomly goes to the floor

Post by OnTheFritz »

So, to update and complete the story, I did end up ordering the bigger bore E32 735/750 master cylinder a couple weeks ago and installed last weekend. The difference in feel is staggering: I hadn't noticed this much before, but in the last couple months of driving, it became increasingly obvious that the pedal would gradually sink while sitting at a red light or similar, especially on my last drive before replacing the MC. Thinking about it now, it was actually scary how the car would start creeping forward after sitting a few seconds given the same pedal pressure :shock: Inspecting the old MC after removal, the piston was absolutely rigid in the bore and wouldn't move freely at all (compared to the expected behavior of a new MC having some slight wiggle room by design), and had a bit of a rattle when shaken around, which led me to think the internal return spring had broken and possibly damage other things. Anyway, new MC on, with a bit of a fight, but after a bench bleed and properly bleeding the whole system after installing some braided SS lines, I'm super happy with how everything feels now. And most importantly: no more creeping forward at stop lights! \:D/
So big thanks to all for keeping my wild mind in check. My car and wallet give their thanks
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plip1953
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Re: Brake pedal randomly goes to the floor

Post by plip1953 »

I'm glad you've had a good outcome. Any circumstances in which the brakes aren't functioning properly is always disconcerting to say the least!!

I'm intrigued that you feel that fitting a bigger bore MC is an 'upgrade'. How so? Just in terms of the bore diameter? All else staying the same, the effect of an increase in bore will be that you need to press the brake pedal harder to achieve the same braking pressure in the lines, but pedal travel will bit less.
Last edited by plip1953 on Wed Sep 02, 2020 12:09 pm, edited 1 time in total.
Phil
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Re: Brake pedal randomly goes to the floor

Post by 86_6series »

I installed the same 750 master with the upgraded calipers, no extra pressure needed.
The car always had pretty good brakes, but the upgrade when your on the track really
makes deeper braking possible.
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Re: Brake pedal randomly goes to the floor

Post by plip1953 »

86_6series wrote: Wed Sep 02, 2020 11:31 am I installed the same 750 master with the upgraded calipers, no extra pressure needed.
The car always had pretty good brakes, but the upgrade when your on the track really
makes deeper braking possible.
Ok, but it's altogether different if you install different calipers at the same time.
Phil
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Re: Brake pedal randomly goes to the floor

Post by 86_6series »

Yeah, the larger calipers needed more volume so that makes sense.
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Re: Brake pedal randomly goes to the floor

Post by OnTheFritz »

plip1953 wrote: Wed Sep 02, 2020 7:36 am All else staying the same, the effect of an increase in bore will be that you need to press the brake pedal harder to achieve the same braking pressure in the lines, but pedal travel will bit less.
You are exactly right there, and that is the effect I was going after anyway. I much prefer braking by modulating pressure rather than pedal travel, the former is simply more natural to me. So the effect of the bigger bore, coupled with braided stainless steel lines, make the brakes feel telepathic in that they do and feel exactly how I want and expect. The added benefit, as described by 86_6series, is that you now have the capacity to fit larger piston brakes without risking the pedal going to the floor being unable to move enough fluid. While not in the immediate future, I do plan to go down a bigger brake upgrade, so having the bigger MC is nice peace of mind for when that comes.
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plip1953
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Re: Brake pedal randomly goes to the floor

Post by plip1953 »

OnTheFritz wrote: Wed Sep 02, 2020 6:58 pm
plip1953 wrote: Wed Sep 02, 2020 7:36 am All else staying the same, the effect of an increase in bore will be that you need to press the brake pedal harder to achieve the same braking pressure in the lines, but pedal travel will bit less.
You are exactly right there, and that is the effect I was going after anyway. I much prefer braking by modulating pressure rather than pedal travel, the former is simply more natural to me. So the effect of the bigger bore, coupled with braided stainless steel lines, make the brakes feel telepathic in that they do and feel exactly how I want and expect. The added benefit, as described by 86_6series, is that you now have the capacity to fit larger piston brakes without risking the pedal going to the floor being unable to move enough fluid. While not in the immediate future, I do plan to go down a bigger brake upgrade, so having the bigger MC is nice peace of mind for when that comes.
I guess after what's you've been though recently any fairly firm pedal will feel good lol

It's a while since I looked at moving on my 635 from the standard single piston floating caliper to to a fixed 4 pot. Is it to the 4 pot caliper as fitted to the M635 or something different again? Having quite recently become a lot more knowledgeable (mainly on the theoretical side) about brakes (mainly for race car applications) I'm really interested to know just how different the piston sizing (ie between single floating and 4 pot) really is. My suspicion is barely at all (in total area), but happy to be shown that's completely wrong. Along the way, are any changes made to the rear brakes?

How about pads? I've been amazed at how different the general feel and modulation capabilities can be just by switching pads.
Phil
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