ABS pump causing brake softness at track?

[Redd]

Does anyone know in detail how the ABS/DSC system works under normal (non-ABS) braking?

Problem - At track, after a few laps in a session, brake pedal goes very soft and pedal travel increases dramatically. Brakes still bite but the pedal is almost at the floor. It is almost identical to very bad brake fluid fade, except for the next part. During the cool down lap of the same session, pedal pressure returns to normal almost immediately. It's unlikely fluid has cooled so rapidly if it is fluid fade. Car is in MDM mode.

Thoughts - Pedal pressure returns when I'm not pushing the engine. I wonder if this pedal pressure could be vacuum related? Or somehow the ABS control unit is leaking pressure somewhere internally? ABS is original from 2009 car, mileage at 110k kms. There are zero errors when this happens. This only happens at track. When I drive hard on road or hill climbs I don't have this issue.

Other details - Brakes are Brembo GT, less than 2 years old. Brake fluid is fresh (less than 3 months) and bled properly by cycling the ABS pump using the diagnostics computer. Brake booster is less than one year old. Brake master cylinder is less than 2 years old. Brake pads are Project Mu HC+ which are known to give very good pedal feel and have plenty of pad left. Steel braided hoses done. Ti brake shims done.

Anyone have any clues what could be cause? Could an internal ABS pump leak cause this symptom? Or might it be vacuum related? Someone has suggested I try to manually activate the ABS a few times to see if it clears out any air bubbles in the system.

Wanted to get some insight before I try to replace the ABS pump which is super expensive.

[Redd]

Brake fluid is Endless RF650. It's a track fluid.

What's the best way to get air and moisture out of the system? I've bled the system at least 8x in the last 2 years as part of the process of replacing certain brake system components and troubleshooting this issue.

[Redd]

Thanks for that. Will doing that also remove any potential bubbles inside the ABS pump?

[Redd]

Thanks. Will try that first. Waiting for a rainy day so I can go out and activate ABS a few times.

[Pig Farmer]

One other thing I did not see mentioned above... Make sure the engine is running when you do the two person method. Goes without saying that you want to do this in a well ventilated garage with fans running. If you're catless, this job is a real treat, but it works better than anything I have discovered. IMHO, Castrol SRF is the way to go.

[SYT_Shadow]

Originally Posted by Pig Farmer

One other thing I did not see mentioned above... Make sure the engine is running when you do the two person method. Goes without saying that you want to do this in a well ventilated garage with fans running. If you're catless, this job is a real treat, but it works better than anything I have discovered. IMHO, Castrol SRF is the way to go.

I also use SRF

Although I'm fairly handy around cars, every task I do not have to do regularly reduces the chances of breaking something

The less things you touch the better

[scotty_miller]

Originally Posted by Redd

Thanks for that. Will doing that also remove any potential bubbles inside the ABS pump?

The proper way to bleed your abs hydro unit is to hook it up to a bmw computer if you have access to one or the software and cycle the abs pumps that way. If not take it some place like a grassy field where you can let the front lock up and the rear still rotate to mimic an abs scenario. I also did it once when I had my 135 where I put the car on jack stands (all four wheels up) and put it in gear and just let the abs cycle that way which worked for me. If none of those above methods seem to fix your issue. Your hydro unit may need to be replaced

[Richbot]

I just got my car state inspected and the car happened to be asking for the biannual fluid change standard BMW service interval change/flush. Had the dealer go ahead and do their procedure and the pedal is markedly better. Granted I haven't bled the brakes in forever but it was nice to feel an improvement...or terrible that I was so lazy before now

I wonder if having them run the flush procedure at the dealer might eliminate a variable if you haven't done that already? EDIT: Nevermind you did that already

Also, I thought the ITBs (and the lousy vacuum you get because ITB's) meant vacuum assist comes from an electric pump on our cars? Maybe that is failing or not able to keep up with demand, or there's a leak developing when hot? http://www.realoem.com/bmw/enUS/show...diagId=34_1627 #8
Not sure if that could cause your symptom but it's a system you haven't thrown parts at yet,

Unfortunately I think if you're using MDM the most likely culprit is going to be the ABS system, MDM uses the ABS pump quite a bit to get the job done, and you may just be a victim of an outlier premature failure, but hey who knows maybe it's something really dumb and cheap you just haven't checked yet

EDIT again: I think if it was vacuum related the pressure sensor would trip a code? Realoem diagram has a vacuum pressure sensor in-line to the booster

[Redd]

Brake vacuum hose with pressure sensor replaced in the last 12 months. I don't think it's a vacuum issue cos loss of vacuum will result in a hard pedal.

Anyway I have a replacement ABS pump in order. Let's see if that helps solve the issue. Will report back with results.

[dparm]

MDM can cause additional heat in the brakes, yes. It will subtly tap the brakes of individual wheels or even drag them a bit to keep the car on the correct line. Sometimes you can smell it.

This can lead to fluid fade because the brakes are being used way more than you think they are.

[roastbeef]

with the brakes and mods you have to the overall braking system, i'm a little skeptical it is fade, even with mdm on a tough track. i don't know much about brake boosters or master cylinders and the role the vacuum hoses on the system, but it sounds like more of a system malfunction rather than insufficient braking ability.

[Redd]

Originally Posted by roastbeef

with the brakes and mods you have to the overall braking system, i'm a little skeptical it is fade, even with mdm on a tough track. i don't know much about brake boosters or master cylinders and the role the vacuum hoses on the system, but it sounds like more of a system malfunction rather than insufficient braking ability.

That's also my thoughts. Cos on my cool down lap, brake pressure returns. Real brake fluid fade doesn't recover that quickly. Anyway, I have an ABS pump on order. I hope this fixes it.

[VictorH]

One other thought, could it be a bad master cylinder?

[Redd]

Brake master cylinder is less than 2 years old. I've been troubleshooting this symptom for 2 years now and replacing the MC hasn't fixed it. Replacing the booster helped a bit but it still occurs at track.

[Redd]

So, I installed a used 27k mile ABS pump today. As suspected, it was the ABS pump causing the pedal softness.

This is what the old ABS pump looked like:




As you can see, the old ABS pump is leaking at the pump interface. The brake fluid has corroded the cover and caused the paint to bubble.

After installing the new ABS pump, braking performance just on street driving is so much better! I suspect the old pump leaked pressure under hard braking, so I was only getting about 60-70% of full pressure. Can't wait to take it to track to test again.

So, if you have brake pedal softness and have tried everything else and nothing works, check the ABS pump.

[roastbeef]

Thanks for the follow up. Glad it's resolved.

[kyrix1st]

Can air in ABS pump cause abs/dsc/rpa light to pop up?

When these lights are gone (intermittently), I hear "thump" sound every time I come to a stop. (maybe ABS pump trying to go work but is gummed?) When I flash traction light on purpose these lights come up again, so I am quite confident it has to do with hydraulic pump... (5DF0, 5DF1 on Carly).