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I am working my way through rebuilding my brake calipers on my 79 528i. Almost all pistons were frozen. I have removed them from the calipers with the help of a diesel injector pop tester. As I am trying to get them cleaned up I have a couple of questions for those of you with more experience;

A) There is a lot of rusty looking sludge behind the pistons. How do I evaluate whether they are too corroded to rebuild?

B) I feel like I read somewhere that you shouldn't remove the bolts that hold the two sides of the caliper together. Removing the bolts would make it a whole lot easier to clean the piston bores and polish them. Is it ok to crack them open?

I also found that the front driver's side wheel bearing grease had degraded to something I can best describe as a super sticky tar. So I am rebuilding the wheel bearing as well. What's the recommended wheel bearing grease? Synthetic? It must get really hot when braking. I assume the last person to do the wheel bearing used the wrong grease with too low of a temp rating.

Any help is appreciated! Trying to get to the Vintage (and back) without a trailer or a tow bill this year!

TNmtnBoy wrote: Mon Mar 16, 2026 9:10 am I am working my way through rebuilding my brake calipers on my 79 528i. Almost all pistons were frozen. I have removed them from the calipers with the help of a diesel injector pop tester. As I am trying to get them cleaned up I have a couple of questions for those of you with more experience;

A) There is a lot of rusty looking sludge behind the pistons. How do I evaluate whether they are too corroded to rebuild?

Look at where it seals, that's what counts. IIRC it's the piston that's the critical part.

B) I feel like I read somewhere that you shouldn't remove the bolts that hold the two sides of the caliper together. Removing the bolts would make it a whole lot easier to clean the piston bores and polish them. Is it ok to crack them open?

Yeah, I've seen that too. Ignore it. Last kits I got even came with seals to bolt the two halves together.

I also found that the front driver's side wheel bearing grease had degraded to something I can best describe as a super sticky tar. So I am rebuilding the wheel bearing as well. What's the recommended wheel bearing grease? Synthetic? It must get really hot when braking. I assume the last person to do the wheel bearing used the wrong grease with too low of a temp rating.

Just get something that says it's for disc brakes.

Thank you, Mike W. !

I wouldn’t try to save corroded pistons. BMW shows rebuild kits including the pistons as available but check with your dealer to be sure. Or you could save yourself lots of time and contact Ireland Engineering and ask if they still offer rebuild services. I believe the issue with splitting the halves is that the bolts are one-time use and not available. I found that certain 911 brakes use the same bolts and are available if you really, really, really want to rebuild them yourself.

LaJolla Independent should be able to get you a rebuilt set if you want them.