Is a hard brake pedal before startup normal?

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HayekFan
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Is a hard brake pedal before startup normal?

Post by HayekFan »

Having brake issues and am noticing the pedal is hard when I go to start the car. As soon as the engine starts the pedal becomes normal, i.e. soft and springy, but it initially it's hard. Is this normal? Or should the brake bomb hold pressure so that the pedal is soft even before the motor starts? I seem to remember it being soft initially but I'm doubting myself.

While driving, the brakes feel normal with no hard pedal during sudden stops. IOW the bomb, which I replaced about 1k miles ago, seems to be working normally. When I park the car and turn off the engine, the pedal stays soft for at least a few minutes. During this time it takes about ten pumps of the pedal for it to get hard. Again, pretty normal.

But if the car sits for longer than that, like say if I go into the grocery store, then when I come back the pedal will be hard. So it's like the brake bomb is trying to hold pressure while the engine is off but is being bled down. Is this the normal behavior or should it hold pressure more or less indefinitely?

What's prompting the question is a weird episode yesterday where the brakes spontaneously started to drag, all four of them. It was like I had my foot lightly on the brake but of course I didn't. As this was happening the pedal was high with no free play at all, but not hard. When I got out to inspect the car all four wheels were extremely hot and smelled like overheated pads.

I managed to limp the car home and parked it with the brakes still dragging. This morning the brakes had relaxed back to normal and the car rolled freely as it should. Throughout the day today the dragging brakes didn't return but I continued to notice the hard pedal before startup if the car sat for awhile.

If such a hard pedal is NOT normal, then I suspect a problem with the brake bomb and/or the booster. If however it is normal, then I'm thinking something must be going on with the master cylinder, specifically a clogged or sticking pressure return port or valve.

Any thoughts or insights appreciated. Car is a 1986 535i 5-speed with anti-lock brakes.

-thanks
Mike W.
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Re: Is a hard brake pedal before startup normal?

Post by Mike W. »

No, the accumulator should hold pressure for a while, I got an ATF bath one time when it had been sitting overnight and thought I had pumped the pedal long enough before changing one of the pressure switches. But given your description it makes me wonder if the M/C is preloaded just a little bit, it would account for the symptoms. You should have a little bit of free play in the pedal.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Thanks Mike. The bomb is holding pressure for a short while, like a few minutes after the engine is off. During that time it takes ten or so pumps to bleed it down to a hard pedal. But it's not holding pressure for much longer than that. For instance if I park the car to get groceries it'll have a hard pedal when I get back. You seem to be confirming that that's not the normal behavior, which is what I thought I remembered but wanted to check.

If this were the only symptom then I'd suspect an internal leak in the hydro boost system, like maybe a slight leak in the membrane of the brake bomb (which I recently replaced) due to a manufacturing defect or something. I would also be suspicious of the servo, but that thing is expensive so I'd be avoiding that thought.

But the brakes locking up brings the master cylinder into the picture, especially if the MC is capable of getting into a preload condition where it can draw down the pressure in the booster system via the servo somehow while the engine is off. Does that seem possible?

I had the brakes lock up like this in another car, including the high pedal and no freeplay, and replacing the MC fixed it. The car was a Toyota with vacuum boost so it was sort of a different scenario but I might just go ahead and replace the MC in the E28 since it's the original and probably due anyway and I can't think of a good test to totally isolate the MC to rule it out.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Well I've got a new brake master cylinder coming from AutohausAZ.
thrty8street
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Re: Is a hard brake pedal before startup normal?

Post by thrty8street »

HayekFan wrote: Aug 12, 2022 9:34 PM Well I've got a new brake master cylinder coming from AutohausAZ.
Was the issue a failed MC?
Blue Shadow
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Re: Is a hard brake pedal before startup normal?

Post by Blue Shadow »

thrty8street wrote: Aug 13, 2022 11:34 AM
HayekFan wrote: Aug 12, 2022 9:34 PM Well I've got a new brake master cylinder coming from AutohausAZ.
Was the issue a failed MC?
We might have to wait for the MC to be delivered and installed before we get the answer to this one.
Federico
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Re: Is a hard brake pedal before startup normal?

Post by Federico »

The MC doesn't have anything to do with the hydro boost losing pressure. The bomb is probably fine too; when the diaphragm goes it will hold no pressure at all. I would try replacing the orings inside the booster, since it may be leaking through the old seals. I think JCS sells a kit, or you can get them as regular metric orings, plus the shaft seal on the MC side.
Also make sure there is some free play at the pedal.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Thanks Federico. I would tend to agree they're not related except that the timing is suspicious. The hard pedal at startup began immediately following the brake lockup episode, so they seem related.

I'm assuming the lockup is a master cylinder problem since it's a classic MC symptom. And then I'm guessing the MC is inducing the pressure bleed in the hydro boost. I'm picturing the MC failing to fully relax back to its neutral position thereby holding pressure on the servo so that its valving lets a little fluid pass. Does that seem plausible? I could also see it being the other way around, with a faulty servo leaking internally and also causing the lockup by spuriously putting pressure on the master cylinder so that it applies the brakes. Does that seem plausible?

I agree it's probably not the bomb for the reason you mentioned, which means it's coming down to the servo or the MC I think.

thrty8street, the MC arrives next week so still a ways off from seeing how this wraps up.
Federico
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Re: Is a hard brake pedal before startup normal?

Post by Federico »

The dragging brakes could also be caused by the booster cylinder. I had it drag on my brakes before, but I fixed it before it became really noticeable.
Inside the booster is a large piston that does the assist, and inside it is a smaller 2-piece control piston which controls the fluid flow.
The pedal moves the small piston relative to the main one, and this does the valving for the assist.
Therefore, if the control piston has leaks, it could feel like you are riding on the brakes.
It may be more noticeable the hotter the fluid is.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Interesting info and especially the fact that you've had the servo make the brakes drag. That's got me thinking the servo is going to end up being the problem here.
Ju@n
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Re: Is a hard brake pedal before startup normal?

Post by Ju@n »

MCs had a common issue where the front piston would not return and make the front brakes drag, had that happen to me, diagnosing that was a B!tch.
Still, not what you mention, and I would agree with Federico that a bad bomb either works or not. Pressure has to be going somewhere, look for leaks in the hydro boost circuit.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Yeah Ju@n I think a leak in the servo is most likely at this point. I'm hoping to pull it tomorrow and then I'll send it off to JCS for a rebuild. I even considered cancelling the MC order but I think I'm going to go ahead and replace it while I'm in there since it's probably due anyway.

I will mention that I've driven the car several times since the brake lockup and it hasn't happened again. I have a feeling that that was a one-time episode associated with the moment failure of the O-ring or whatever it was that failed in the servo. I imagine there was a unique set of conditions that arose during that event that caused the lockup. My guess is it won't get into that state again and instead has settled into a "quiescent mode" where it'll just do the hard pedal at startup behavior now.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Well, the plot thickens. Looks like the leak is in the power flow regulator. I hadn't even considered the PFR until I checked the Bentley manual earlier today.

Bentley has a test on page 13 of the Brakes section (steps 6 and 7) to tell if loss of pressure in the hydro boost system is due to a leak in the servo or the PFR. What I'm seeing is that shortly after turning the engine off and while the boost system is still holding pressure*, there's a substantial flow of fluid coming out of the PFR's return line port if I disconnect the hose (which goes to the reservoir). According to Bentley this shows the leak path is through the PFR and not the servo, which makes sense.

A new PFR is almost $900 at AutohausAZ so it looks like I'll probably be trying to source a good used one.

I do wonder if the pressure switches can cause an internal leak in the PFR. I wouldn't think so but does anyone know?

*I timed the leak and it takes about ten minutes to bleed down. It's fast enough that you can actually see the fluid level rise in the reservoir.
Ju@n
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Re: Is a hard brake pedal before startup normal?

Post by Ju@n »

Great! you've narrowed it down it seems, although I don't think I'd buy that new if it is 900 USD :laugh: try to get it second hand :)

PS: I have both sensors leaking, it does take QUITE a while for the pressure to go down, but it does in a couple days. Have both new ones waiting to go in :)
1st 5er
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Re: Is a hard brake pedal before startup normal?

Post by 1st 5er »

Glad I subscribed.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Well I found a good used PFR for about $150 shipped and ordered the two switches from AutohausAZ.

With the leak now shifting from the servo to the PFR, I'm more puzzled by the brake lockup episode and how/whether it's related.
gadget73
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Re: Is a hard brake pedal before startup normal?

Post by gadget73 »

Is it possible to take the PFR apart? If so I'd be real tempted to take apart the old one just to see if there was anything to learn from it. Guessing you'll find wear on the parts that allow for some sort of a leakage path. Thats the usual way hydraulic things fail, dirty fluid causes scratches on the machined surfaces and the very tight tolerances become not tight enough.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Well yesterday I replaced the PFR which seems to have fixed the hard pedal at startup. Whereas the hydro boost had only been holding pressure for 10 minutes or so after turning off the engine before, this morning the pedal was normal meaning it held fine overnight. So at this point I'm going to say that issue is solved.

But to my surprise I've had another episode of the dragging brakes. Not as bad as before but that's probably because I was closer to getting home when it happened so it didn't have time to build to the same level of lockup. So I'm back to suspecting the master cylinder again. The MC I ordered from AutohausAZ has arrived and I'll probably try to install it later this week.

BTW something I've noticed is that just before the lockup starts to become noticeable, the 50 mph upper control arm shimmy gets going without my having to touch the brakes. That's its "tell."

Oh and it doesn't look like there's any disassembly I can do on the PFR to see what might've failed internally. There's a big metallic disk that appears to be pressed into the housing which I'm guessing is the base of the main valve assembly or something along those lines. It would be interesting to pull it out and take a look but unfortunately I don't have a good way of doing that.
Ju@n
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Re: Is a hard brake pedal before startup normal?

Post by Ju@n »

Had that same issue with the brakes dragging, the shimmy at 50-60 mph is horrendously present, but in my scenario, only the front brakes were dragging, and it's a common occurrence with the stock MC. Switched to a e32 one (also added "big brakes") and that hasn't happened anymore.
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

Fronts were definitely hotter than rears for me this time. I couldn't keep my hand on the front wheels but I could the rears. Last time I may have been mistaken in thinking it was the rears too although I don't think so. Is the e32 MC a direct replacement?
Ju@n
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Re: Is a hard brake pedal before startup normal?

Post by Ju@n »

HayekFan wrote: Aug 24, 2022 5:24 PM Fronts were definitely hotter than rears for me this time. I couldn't keep my hand on the front wheels but I could the rears. Last time I may have been mistaken in thinking it was the rears too although I don't think so. Is the e32 MC a direct replacement?
Yes, at least I don't remember having to do anything special to fit it :)
You'll get a "shorter" pedal, although, I could barely notice it
HayekFan
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Re: Is a hard brake pedal before startup normal?

Post by HayekFan »

I installed the new brake master cylinder and am reasonably confident it has fixed the dragging brakes issue. No signs of brake drag after driving the car extensively today. The last couple times I did this much driving the brakes dragged but not this time. If something changes I'll post an update. Very glad to have this one seemingly solved and thanks all for the comments.
Ju@n
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Re: Is a hard brake pedal before startup normal?

Post by Ju@n »

HayekFan wrote: Aug 25, 2022 10:42 PM I installed the new brake master cylinder and am reasonably confident it has fixed the dragging brakes issue. No signs of brake drag after driving the car extensively today. The last couple times I did this much driving the brakes dragged but not this time. If something changes I'll post an update. Very glad to have this one seemingly solved and thanks all for the comments.
:) :) :) :)
Glad to hear that!
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