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Hello everyone! It's been a minute since I've posted here. My E24 M6 (with S38B36 swap) has a brake problem. Searched high and low and can't find an obvious answer. I first noticed this issue just before the car got put away during the COVID lock down. I made a note from my research at the time saying "change the brake master cylinder!!!". Car then sat for two years while life happened. Now back on the case with a plan to bring this car back and make it a daily again.

Symptoms:
Brake pedal goes soft when the engine is started. Minimal effective braking and pedal goes to floor with little pressure. After shutting it off, a few pumps and the pedal is firm like a rock and stays so. In the process of diagnosis, I looked at the accumulator. It discharges PS fluid back to the PS reservoir (observed by level rising) when the brake is pumped with the engine off as the pedal hardens. And the PS reservoir level dips back down when you start the engine again (which I assume is the accumulator recharging).

Steps taken:
- Changed out the cruddy old OE brake lines with steel. The left front didn't even bleed so suspected the line was cruddy. Side note: I had to chop the fronts at the hard line as connections had fully seized and no amount of heat was going to break them loose. I learned how to make bubble flairs and bend some small extensions. All good. No leaks. Rear lines went in easily. However, changing the lines had no impact on the brake symptoms.
- Changed the brake master cylinder (put in the 540 version as I have 540 brakes). Sadly, no difference in symptoms. Still soft when engine running.
- Flushed lots of brake fluid through using motive power bleeder. Nothing but golden brake fluid comes out now. No change.
- PS fluid level was a little low. Flushed ATF out of PS reservoir and put in Pentosin CHF 202. Set to 10mm below rim with accumulator discharged. Moved lock to lock. No difference in brake symptoms.

May or may not be relevant:
- The clutch slave WILL NOT bleed. Tried pushing fluid from above. Tried pushing fluid from below. Tried some compressed air to see if I could blow it out. Nada. Clutch still works though! I think the lines are vintage 1988 and probably blocked on both ends. However, I don't think this is causing the issue for my brakes. Just another problem to fix!
- The PS pump is whiny and has been from time to time for quite a long while. It works. Sometimes there's a hiccup, most often near full lock but otherwise it works.
- Repeated pumping of brakes to try to get a firm pedal with engine running seems to impact idle??? I can't be certain that it correlates or is just coincidence and not sure how or why this would happen but as I pump the brakes with the engine running, the tach sometimes drops or rises seemingly in response. Could an issue in the PS pump cause drag on the engine?
- Brake lines on this car are custom. As result of the engine swap the ABS pump had to move to the D/S. Some of the PS lines are also custom (B35 pump on a B36 motor).
- Accumulator and brake pressure switch replaced in 2011.

Questions:
- Could the accumulator be malfunctioning even if it seems to flow to and from the PS reservoir as expected?
- How do I diagnose brake pressure switch and oil pressure switch? Still not clear what they do.
- What else do I need to look at? It has to be something that changes with the engine on as the brakes firm right up when it's off.

as someone not specifically familiar with the brake system on this car, this honestly sounds like air somewhere in the system. I've worked on things with non-factory brakes that had to have a caliper unbolted and rotated to some odd position in order to get the last little bubble out or it just would not work.

might also try the old fashioned pump and crack the bleeder valve method. I've had things that didn't respond to power bleeding but bleeding brakes like it was 1940 did the job.

then again I've also done this on a car and it turned out my new master cylinder was also bad.

Not sure how much help, but I'll take a stab.

ianwood wrote: May 01, 2023 8:22 PM Symptoms:
Brake pedal goes soft when the engine is started. Minimal effective braking and pedal goes to floor with little pressure. After shutting it off, a few pumps and the pedal is firm like a rock and stays so. In the process of diagnosis, I looked at the accumulator. It discharges PS fluid back to the PS reservoir (observed by level rising) when the brake is pumped with the engine off as the pedal hardens. And the PS reservoir level dips back down when you start the engine again (which I assume is the accumulator recharging).

On my 535i I always had a soft pedal, but rock hard when off and I discharged the accumulator. Bled it, did front hoses in time, rebuilt at least the front calipers and maybe the rears, to no change. But the brakes were fine, good stopping power, no issues there.

Steps taken:
- Changed out the cruddy old OE brake lines with steel. The left front didn't even bleed so suspected the line was cruddy. Side note: I had to chop the fronts at the hard line as connections had fully seized and no amount of heat was going to break them loose. I learned how to make bubble flairs and bend some small extensions. All good. No leaks. Rear lines went in easily. However, changing the lines had no impact on the brake symptoms.
- Changed the brake master cylinder (put in the 540 version as I have 540 brakes). Sadly, no difference in symptoms. Still soft when engine running.
- Flushed lots of brake fluid through using motive power bleeder. Nothing but golden brake fluid comes out now. No change.
- PS fluid level was a little low. Flushed ATF out of PS reservoir and put in Pentosin CHF 202. Set to 10mm below rim with accumulator discharged. Moved lock to lock. No difference in brake symptoms.

I've heard of, and run into on other cars, where the ABS had to be activated to bleed it well, there would be air trapped inside the ABS unit. Yours repositioned may be a problem, lines may go up and down in a way the factory ones didn't and create a bubble you're not getting out of there.

May or may not be relevant:
- The clutch slave WILL NOT bleed. Tried pushing fluid from above. Tried pushing fluid from below. Tried some compressed air to see if I could blow it out. Nada. Clutch still works though! I think the lines are vintage 1988 and probably blocked on both ends. However, I don't think this is causing the issue for my brakes. Just another problem to fix!

BMW slaves bleed oddly. There's several techniques that work, but the traditional one of pump and bleed does not work. I remove the slave, pump the pushrod a few times, never opening the bleeder, reinstall, never opening the bleeder and pump the pedal. Takes some pumps, but it works. Do no open the bleeder!

- The PS pump is whiny and has been from time to time for quite a long while. It works. Sometimes there's a hiccup, most often near full lock but otherwise it works.

A somewhat unusual, but not unheard of problem is the bolts holding the pump together aren't tight. Not loose and they don't have to be crazy tight, but tight. Drove me crazy for a while, assuming yours is the same design. Not overtorqued, but tight. Also the P/S pump belt on my 535 had to be very tight not to slip in the rain. Felt too tight, but that's what worked.

- Repeated pumping of brakes to try to get a firm pedal with engine running seems to impact idle??? I can't be certain that it correlates or is just coincidence and not sure how or why this would happen but as I pump the brakes with the engine running, the tach sometimes drops or rises seemingly in response. Could an issue in the PS pump cause drag on the engine?

Well, it is working the P/S pump.

- Brake lines on this car are custom. As result of the engine swap the ABS pump had to move to the D/S. Some of the PS lines are also custom (B35 pump on a B36 motor).
- Accumulator and brake pressure switch replaced in 2011.

See above on brake lines. Accumulator is getting old, if not old and done, but how many pumps with the engine off to get a hard pedal? Should be at least half a dozen. Brake pressure switches have nothing to do with the operation, they monitor pressure only, but the system is hydro, not electric.

Questions:
- Could the accumulator be malfunctioning even if it seems to flow to and from the PS reservoir as expected?
- How do I diagnose brake pressure switch and oil pressure switch? Still not clear what they do.
- What else do I need to look at? It has to be something that changes with the engine on as the brakes firm right up when it's off.

Accumulator failure mode is regular casual braking, then feeling like there's a brick under the pedal in a panic stop.
Pressure switches. They will set off a light, but have no effect on braking. They're mostly there to leak as they age.
I suspect you either have air trapped in either the ABS or the custom lines, and or a pad/rotor issue.

OK. Thanks for the feedback. I will bleed it again and report back.

A few extra notes. The brake light engages when the ignition is on but engine not started and goes out pretty much immediately on start up. Pumping the brakes after the engine is turned off is about 10+ pumps before the pedal goes rock hard. So all indications suggest the accumulator is doing something.

For bleeding, this time I'll do the 2 person pump and bleed and will let it flow a bit (topping up the reservoir after each corner). The motive bleeder has not let me down before but that has always just been after work on the calipers. New MC and changing brake lines is admittedly a fair bit more. If there's air in there, I'll get it out.

Think about getting on something loose, at even modest speed, but not too modest, and hammering on the brakes. See if you can put it into ABS mode to activate the internal parts. That may aid in bleeding.

Mike W. wrote: May 04, 2023 11:04 PM Think about getting on something loose, at even modest speed, but not too modest, and hammering on the brakes. See if you can put it into ABS mode to activate the internal parts. That may aid in bleeding.

I've read a bunch of posts about actuating ABS to bleed these cars. Someone even built a whole switch panel that would activate the different corners. I studied the wiring diagram and could mock up something similar to trigger ABS on the different lines. Seems a bit much. And what I am not sure I understand is why that's even necessary. Maybe the ABS valves opening and closing agitates trapped air and gets it moving down the line better. But ABS is open by default. Push hard enough and the bubbles should just pass through. Unless I am missing something.

Anyway, after two good ole foot on the pedal bleeding cycles, the pedal feel did change a little. Not much air came out though. It's definitely not resolved but the pedal doesn't go all the way to the floor now with the engine running. And weirdly, I am getting micro bubbles on my P/S front line. Could be I cracked it too much and sucked in some air at the nipple but will check the line to be sure. It didn't have any micro bubbles with the Motive earlier.

Has anyone used one of these:

https://www.amazon.com/Mityvac-MITMV850 ... B0002SQYUA

I think between that and the Motive, you can push and pull at the same time and give the bubbles a harder time of lingering. And if that doesn't do it, you can also use the Mityvac to reverse bleed. Added benefit is you can see the pressure at each corner. Also might be what's needed for the clutch. I'll remove the slave if I have to but will likely get this and try reverse bleed first.