Front Calipers not Releasing

[johnjmurray]

All:
Thanks for all of your help in the past. This forum has really enabled me to elevate my skill set.

I'm at a loss. The front calipers are not releasing at rest. The rears seem fine/normal. I've replaced the hoses and calipers (rebuilt). Bled the system many time. Activated the ABS pump along the way (INPA). This problem seems to be upstream of the wheelhouse. My suspicion is the seal in the master cylinder. I've tried the brass bushings with no success.

Have any of you crossed this issue before? I don't want to replace the MC if I don't have to. The rebuilt calipers were somewhat incomplete on arrival (Centric brand but BMW caliper). They were missing some of the incidental parts that normally come with but since both boxes were identical, I passed over that. I had spare parts anyhoo. I suspect that they were returned to the vendor and I am the next owner. Compressing the caliper with the bleeder open, they squish just fine with good flow out. I don't think the calipers are defective but I'm open to your suggestions.

Thanks in advance,
Johnny

[jclausen]

If you had a locking caliper you would loosen the hose to see if the pressure gets released. Since the calipers and hoses are new try loosing the pipes at the next connection and see what happens. You may have a bad master cylinder

[Honolulu]

Johnny: this is a problem that comes up mostly on older bimmers. What's going wrong (on those cars) is that the flex line to the caliper is failing internally and swelling near shut. When you step on the brake, the power assist sends high pressure brake fluid to the caliper, and you stop. However with a clogged line, there is no similar high pressure return, and the caliper stays shut. My suggestion is this: jack up the car, put it on stands. Take off one or both front wheels. Run the engine and step on the brake pedal firmly. The wheels should be locked, go and check them. If they're locked, loosen the caliper bleeder bolt and release the pressure. Not much will come out. Solution, if confirmed, is to replace the flex lines.

Alternatively, if you've messed with the master cylinder or the rod between the MC and brake pedal, make sure there is just a little free play there. The piston in the master needs to fully retract (it's spring loaded) for the fluid to return.

At worst, the brake caliper pistons are stuck from corrosion, or cocked in their bore(s). Caliper rebuild isn't rocket science, and rebuild kits aren't expensive.

Lotsa luck, let us know how it goes.

[dhurley34]

I concur with the hoses being bad.

[johnjmurray]

Thanks guys. The hoses are brand new SS from ECS. I've seen the threads about hoses going bad internally. I didn't disassemble the rebuilt calipers prior to installation. I suppose it's possible they could have corroded on the shelf.
I like the idea of free-play in the MC piston. Vacuum booster seems fine but I get a load of sucking if I pull the check valve. This does not have any effect on the condition at the rotors. Neither does cracking the bleeder. I can only get spin if I compress the cals with a c-clamp.
ABS seems working properly. The pedal seems a bit too soft compared to my other (2005) 330.
Thanks and keep it coming.

[MarcoZandrini]

I’m thinking it’s tge master cylinder. Why? The pistons move freely and the hisss are new. What else?

[Junglenut]

master cylinder should be good if when the engine vaccum hose to master cylinder is detached after engine off a release of pressure occurs.

[johnjmurray]

I pulled and plugged the booster and the calipers were still in full contact after a test drive. I'm thinking it's the MC too. Thanks for all your insights.
Johnny

[randy72877]

If the brakes don't release when you open the bleeder screws the problem is NOT hydraulic. If there was residual pressure in the brake hydraulics causing the brake to stay on, opening the bleeder screws will release it and the rotors should turn freely. It is probably a mechanical problem. Do the pads slide freely in the caliper bracket? Are the slide pins clean and free? They should be lubed with silicone based grease (petroleum grease will destroy the rubber) make sure that you have the correct pads and that the caliper pistons return easily. Good luck.

Sent from my SAMSUNG-SM-G870A using Tapatalk

[johnjmurray]

Thanks, Randy. I was avoiding greasing the pins based on the caution we see on this forum. I do have that type grease. What the heck, may as well try it. I'm pretty sure they are actual BMW pins. This has been an issue for a number of years. I just never had a basis for comparison before. Now it's an issue <g>.

[gm75]

If the pressure release at the bleeder didnt retract it then the caliper is defective. Probably the piston seal. You can take it apart and rebuild or just return that junk.

[johnjmurray]

Thanks GM. This issue has been present through 2 sets of calipers. It's only the fronts, rear wheels rotate as normal. I think there's something going on in the master cylinder for this reason. I did try greasing the pins without success. The new MC is in the house so I have my work cut out...

[gm75]

did you test the piston? did you pull it out and check the seals?

[Thaniel]

did you test the piston? did you pull it out and check the seals?

I agree. Been my experience that the caliper pistons start to rust (behind the rubber accordion boot) When they get shoved back in for new pads they jam in the seals and start to stick. A simple rebuild with new pistons would fix it. but sadly I've never found ANY place that sells the pistons (seals yes). So rebuilt calipers is the next best option.

Thaniel

[Cobra1956]

o ring inside the piston could be bad. It is square cut and the rocking motion is what takes the pressure off the pads when you release the brake. Easy fix . Did not read everything, pin dry or greased? Try this first

[johnjmurray]

Thanks all. the original problem was sticking caliper so I replaced them. Noted that the exact same problem continued after replacement. It was hard for me to consider that the 'new' rebuilt caliper was failing in the same way as the cruddy old one. This lead me to conclude the trouble was upstream. After replacing all the other upstream components, including the MC, I'm back to the idea that defective calipers were delivered. Ordered a couple rebuild kits. They should arrive soon. I only have the weekends to mess with this car; it's my daily driver. I will update as soon as I'm finished rebuilding...

[johnjmurray]

Got it. The rebuilt calipers were, in fact, not rebuilt correctly. When I pulled the pistons, each had a gunk ring where the seal hits. Looks like they forgot to lube the seals during installation. My rebuild went netter. The calipers are releasing normally again.
Thank you all fir pitching in to help me.
Johnny

[mdistel]

I also had multiple bad calipers on an e36 so I've returned to rebuilding them. I've had trouble finding ATE kits and used a different brand that wasn't a good fit for my E39, next time I'll have to spring for genuine kits.

Mark