I'm Stumped: Low Brake Pedal

[retrogrouch]

It recirculates the fluid back into the reservoir?

That's a neat idea. I wonder if it acts like air does the clutch system.

A diagram will help explain.

[DYL]

I tried pushing the pedal and then running the pump with the bleeders closed. No change. It was worth a try though. I think the bleeders need to be open when the pump is running. I believe the system dead ends at the calipers, and thus fluid would not recirculate.

[bimerok]

The fluid would obviously not recirculate at the caliper as it only has one line. But ABS unit does dump the pressure to release the brakes while you are still applying pressure at the pedal. That fluid has to go somewhere...

[taylor@bimmertools]

Had a very similar condition with my car. 4 new calipers and master cylinder. Bled properly multiple times as well as ABS unit. In the end the culprit was the booster. Did a smoke test on it and sure enough it was bad. Grabbed a used one and all is right in the world. Car is a 92 325is and the booster came of an e36 m3. I know the p/n are different but it worked fine. Also replaced the brake pedal position sensor. No warning lights and brakes feel 100%. Smoke test is easy to perform and it seems like you're running out of possible leads.

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Also should mention pedal would pump up stiff when car was off then once it was turned on the pedal would go all squishy again.

[DYL]

Hmm, I heard that being able to pump up a firm brake pedal with the car off indicated a good brake booster... Did you have a shop do the smoke test?

[DYL]

Bump, any word from OP?

[Jfriday11]

I've been super busy for the past couple months or so and haven't had any time to work on the car or take it to the dealership, but I should have more answers and hopefully a solution in the next couple weeks as I have to get the car ready for VIR the first week of October.
I may just replace the booster and pedal position sensor just for the peace of mind of having newer brake system components and to see if that helps at all. I'm still going to see about flushing the brake fluid while cycling the abs, but my gut instinct is pretty pessimistic about it working since I've already bled many many times following abs engaging stops.

[DYL]

I think whoever suggested pumping up the pedal and holding it halfway down may be onto something. Noticed that my pedal is a tiny bit squishy with the car off, so it seems reasonable that the booster would amplify it.

[craigporter3]

I was having a similar problem for a while. I ended up tracing it back to the pads themselves. I wasn't torquing down the carrier bolts enough so I was putting an almost imperceptible slant in the pads over the course of a track weekend, but by the end of it the pedal would go about halfway to the floor before you could feel it. I didn't read the entire thread, so I'm not sure its the same problem as OP, but something to think about

[Jfriday11]

I noticed a couple weeks ago while driving on the highway that my front wheel bearings are bad so I bought some new ones that I'll be putting on this weekend and am hoping will fix the brake pedal issue.
I read an article that had this to say about wheel bearings and pedal travel, "Brake rotors are held in alignment by wheel bearings. Slack in the bearings will allow the rotor to move and push the caliper pistons into their bores. When the brakes are applied, the pistons have to travel much further, causing a low pedal. A modern bolt in hub bearing should have no discernible slack. Any movement in the bearings means it is failing and should be replaced." http://www.agcoauto.com/content/news/p2_articleid/266

Fingers crossed that it works!

[DYL]

Hmm my front bearings are making a little noise when they spin, but hardly any play. They aren't bad yet, but I'm keeping an eye on them.

[Massive Lee]

Hmm my front bearings are making a little noise when they spin, but hardly any play. They aren't bad yet, but I'm keeping an eye on them.

Yup. Loose wheel bearings will allow slack in the brake pedal as the discs push pistons back into their bore. After bleeding the pedal would be extra stiff with the car static. But would get loose as soon as th car would move, and espcially when it steers.

I had that problem over 10 years ago and it left me perplex for several hours...

Lee

[lelandr]

I would love to hear a solution, but aside from completely bypassing the ABS unit (e30 ABS is the benchmark of reliability, so I've heard), I believe that I've tried everything.

According to the manual from Bentley (which leaves out a step) & the factory service manual you can manually bleed the ABS with an E30, early E36 or E90 with the following procedure:

- Use a pressure bleeder with under 2bar.
- You will NEED someone to pump the brake pedal.
- Basically, you pressurize the system as if you are self bleeding. However, you should put the key in the ignition & turn it to the on position (do not start the car, just ensure that all of the power distribution is active).
- For E30s & E90s, you open the valve & then tell the assistant to pump the pedal quickly 12 times, holding down on the last pump. Early E36s is 10 times IIRC.
- This will tell the ABS controller to open the solenoids so that the system can be bled.

I chased this problem for a while after rebuilding my entire brake hydraulic system some years ago. Once I figured this out, it solved the problem.

My E30 M3 will always start an event with a rock solid pedal, but many times by the end of the weekend, the pedal will be a little softer. Better than any street car, but not quite where it started. This is using an old set of cool willys, stock rotors & the solid guide bushings.

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Yup. Loose wheel bearings will allow slack in the brake pedal as the discs push pistons back into their bore. After bleeding the pedal would be extra stiff with the car static. But would get loose as soon as th car would move, and espcially when it steers.

Lee

Also, check rotor runout. If the rotors are slightly warped or have inconsistent pad deposits, then you can get the same thing.

How is OP bedding in the pads/rotors? The Stoptech site, IIRC, has a good write-up on properly bedding in pads. With track or race pads, it is going to be hard to do as you really should generate some heat in the pads/rotors together. It would be good to get several decelerations from 60+ down to around 20. After you bed the pads/rotors try not to come to a stop until the components have had a chance to cool down.

[rob22sf]

^^

run out indeed

http://www.centricparts.com/files/te...ation-tips.pdf

[Jfriday11]

Any reason why this procedure wouldn't work for late model OBDII cars? I'm thinking I might give it a try anyway after I replace the the wheel bearings.

[PbFut]

If the pedal is dropping as pads wear, it is tapper. period. Caused by the caliper flexing, the pads wear uneven and then flex and rock in their position when pressure is applied. It's not a lot but requires more pedal as tapper worsens over life of pad. New pads, pedal comes back up and it starts all over as the new set again wears uneven. Correctable flex occurs mostly in the caliper pin bushing area. Brass bushings help but are not a 100% cure. Bushings cured about 85% of the problem for me.

[cooler breeze]

Where does one get the brass bushings?

[DBVille]

Where does one get the brass bushings?

Bimmerworld, Turner, and others.

http://store.bimmerworld.com/bimmerw...e-kit-p46.aspx

[MJockey]

Thanks to this thread I was able to fix my low-ish brake pedal.
I connected my pressure bleeder, activated the ABS pump for 15 sec with the bleed screws closed. Then one by one, I open the bleed screws and activated the ABS pump while the screw was open. This was followed by a standard 2 person bleed that I always do. I must of had some air in the ABS system.

[DYL]

Glad to hear it! Hope I can fix my problem too, once I have time to work on it. I tried cycling it and then bleeding, then bleeding it while cycling it, but forgot to pump up my pressure bleeder. Next time, I will actually pressurize my bleeder and also try holding the pedal halfway down.

[Jfriday11]

I finally took my car to BMW this morning to have the brakes flushed while properly cycling the abs for a cool $150 and the pedal seems to be quite a bit better (although still not the super firm pedal I romanticize about, but seems to be an acceptable Oem feel). I still need to replace the wheel bearings and put some fresh pads on tonight so I'm hoping that helps even more.

Hopefully sometime in the next year or so I'll have the finances to get a big brake kit to really get the feel I desire, but I guess this will do for now. Thanks for the help everyone!

[DYL]

This is excellent news! New pads and front bearings can only make it better. On a related note, I read on the DVATP E36 blog that BMW dealers lube the caliper pins with silicone paste. When I worked on my front brakes, the pins were dry and the rubber was a little dusty. I put them back in without cleaning or lubing, following a different diy. I wonder if this caused any problems with my low spongy brake pedal...

[bimerok]

This is excellent news! New pads and front bearings can only make it better. On a related note, I read on the DVATP E36 blog that BMW dealers lube the caliper pins with silicone paste. When I worked on my front brakes, the pins were dry and the rubber was a little dusty. I put them back in without cleaning or lubing, following a different diy. I wonder if this caused any problems with my low spongy brake pedal...

You should always at a minimum clean the pins. These are sliding calipers and should slide on the pins smoothly and freely. I also always lube them too.

[DYL]

Just to clarify, they looked pretty clean already. Since the blog mentioned that the area was totally clean even after wearing through a set of rotors and pads, I'm seriously thinking about picking some up. Only concern is cleaning out old grease

[PbFut]

You should always at a minimum clean the pins. These are sliding calipers and should slide on the pins smoothly and freely. I also always lube them too.

Nope...Lube only causes more problems. Lube softens the rubber and pad taper will result even at moderate pressures. Read the BMW service manual. No lube for a reason.