I just installed a pair of rebuilt BMW front calipers in mt 73 2.0 and the inside pas keep retracting. I have stainless break lines and a 911 master cylinder on it already. The inside pads do contact the disk when I hit the breaks but it docent stay " out" like it should. It is stopping the car but not with the performance I would like to see. Also the problem seems to be improving with time. I think they are bleed well... Any ideas?

If it is improving with driving should I just take it out for a spin and watch it for a day? I have no idea what's happening here and I am getting really irritated.

-Brad

Brad,
Did you have the "ears" of the calipers machine so that they are centered on the disk? How much is the pad moving back? If I understand you correctly, the inside piston is retracting when you let off the brake?

note sure what yoru saying here, the pad contacts teh disc, but doesn't stay out????? are you saying ithe pad retracts on its own whilest braking after it contacts teh pad?????

at anyrate BMW calipers might be bigger, but teh limiting factor is the disc, it can only take so much heat from teh bigger calipers and then you will get brake fade.

I have had great use of stock calipers with porterfeld racing pads, more stopping than you will probebly need.

I would say you have air in the system. Rebleed.

Everything is machined and installed correctly... When I step on the break the pads move in and stop the car but when you take you foot off the break they retract further than they should.

I assume you are using 320I units. The ears need to be machined to center the caliper over the rotor. These still aren't ideal as the pad still ride over the top of the rotors not in the same path as the stock rotors. BMW 2002 4 piston units are better suited for a direct bolt on fit. It sounds like your heading for trouble. Get your stock calipers rebuilt by Eric Shea or get kits and do them yourself ASAP and don't drive the car with those death trap brakes until you do.

What is likely happening is only one pad is pressing against the rotor. Eventually after great stress on the rotor the other pad will walk out far enough to help. But rotors are not designed to bear the side load your applying and could snap off. This type of failure will leave a spinning cleaver that will either slice off the pad on the next rotation of jam in the side of the pad cause total lockup.

Modifying brakes is a dangerous game. There are others that have done this mod with success. Do a search and find the links to the threads. Also, Stock 914 brakes great when rebuilt properly. Replace the rubber lines and rebuild the calipers and you should be good with stock rubber.

If you rebuilt them before you installed them, the seals are new and will have to break in. Check the factory manual for the condition.

I'm working on a complete bleeding and bedding procedure but, it's really multifacited and this is just one of the faces (probably the one that is affecting you).

Click to view attachment

It could also be that you have a sticking knockback pin. This would cause that piston to hang up on the pin and not self adjust out to the proper position. Knockbacks usually push the piston "outward" toward the disk to ensure a high pedal however, if one is stuck on the pin, it could work in the opposite direction pulling the piston back in after pressure release.

This problem can be diagnosed while rebuilding and reinstalling the pistons. We use an arbor press. If the piston does not "bounce-back" by roughly 1/16th of an inch, the knockback mechanism is not operating properly.

Click to view attachment

This situation may also be caused by an out of round rotor, bad bearings or both. Do you have a "thumping" feel when you press your pedal? Extreme "runout" will cause the piston to be forced further back than normal. BMW calipers can "Exasperate" this situation because you have now put a tremendous amount of pad on a smaller solid rotor. That and the fact that BMW calipers are weight pigs is why I really don't like this "mod" (I refuse to call it an upgrade... sorry).

To help you're "overall" condition, you should bed the pads (but again, doing this properly may warp your rotors):

http://www.pmbperformance.com/page/page/1492423.htm

That will help your seals break in and hopefully aleviate your piston's overly agressive return situation.

E.

What size of pistons are in the BMW Calipers?

48

I believe he bought them from AA. Dunno if George machines them or not.

Brad, your car likely needs to be bled better. The last time you left my house they still were not where I would want them to be.

I need brake clean

I did buy the Auto Atlanta kit and I called them this morning. They said they machine them before they ship the parts. It sounds like the sticking pin thing is what I have and when I called Auto Atlanta they said the same thing. They told me to drive it a bit and see if it improves over a few days.

I am really nervous about this issue and am trying to drive slow and safe. It feels like it stops good with no pulling thumping, squealing or rubbing. I just don't like how the piston is pulling back and the pedal feel is a bit soft.

Jon, I have had 3 can of break clean rolling around in my car since the last time I saw you.

If you wouldn't mind I'd love to stop by and have you take a look at it.

-Bard

Sunday I will be at home most of the day. Come on by and bring a bag of rags as payment

I think they probably "won't" be where you want them to be for a week or two. The seals probably need to break in before the pedal will come up. The comment <<the problem seems to be improving with time>> leads me to believe the seals are breaking in.

Again, properly bedding the pads will help as the heat generated will expidite the break-in processs. The problem is, the heat generated can also warp the rotors with that brake combo.

Thanks Eric and Jon,

Jon, I'll call you later and work something out for Sunday. Bag of rags is no problem and I STILL have several cans of break clean.

I have had the calipers in my garage for about a year and Auto Atlanta says they just need to be worked to break them in. Hopefully they are right.

-Brad

-Brad

Kinda scary. I wouldn't want to drive it. In you garage try pushing really hard on that pedal a few times and make that thing piston pop out into its correct spot. I would think that if you did that a few times and it isn't working right it is never going to. This could be a great test of the AA's customer service.

I am running the BMW calipers, and so far, they have been great now that the car is back on the road. On some initial fairly gentle runs, I was making quite firm stops, without even touch the rust on the rears. I really have to lay on them to get the rears to come into play.

yeah, that's not a good thing. You WANT the rears to work with your fronts all the time. You just don't want them to work too much.

When I push the break both sides of the front engage, but the piston on both insides retracts. I drove it again this afternoon and it seems to be getting a firmer pedal. I haven't had the wheel off to look and see if it looks better but I have hit the breaks fairly hard a few times and it stops pretty fast, but no lock-up.

Actually, I am considering sending my 914 calipers off to eric and going back to them. I think the biggest issue I was having previous to the BMW calipers was not having bleed the lines at the p-valve.

Took it out a couple times today... The break pedal has lost the spongy feeling and I was able to lock up both front breaks. Significantly better break performance than before. I had never been able to lock them up before. Haven't pulled a wheel off yet, but I am assuming they just needed breaking in. I'll take a look tomorrow.