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M6 front brake caliper

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M6 front brake caliper

Post by tmoretta » Wed Jan 13, 2021 10:30 pm

I am in the process of rebuilding left front caliper on my M6. The pistons are very stuck - even air pressure will not push two out. Can I separate the halves of the 4 piston caliper for better access? Will I just need to source new o rings?

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Re: M6 front brake caliper

Post by Da_Hose » Thu Jan 14, 2021 12:44 am

Another trick I have heard of people trying is to plug the brake line holes, and replace the bleed screw with a grease nipple (same threads apparently). Then you use a grease gun on the fitting and pop out the pistons with the hydraulic pressure.

As to your first question, splitting calipers is always possible, but in the case of M6 calipers you won't get access to the back side of the pistons. If the grease gun approach doesn't work for you, I suggest just soaking the inside/outside with PB Blaster repeatedly. After a week of sitting in that stuff, I have a feeling things would come loose, and the air compressor approach might work. Although it would be sure to make a mess when things let go.

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Re: M6 front brake caliper

Post by bpoliakoff » Thu Jan 14, 2021 1:56 am

I just rebuilt some early euro calipers and couldn't get the pistons out for the life of me. I took them down to a shop that does clutch and brake supplies/remanufacturing and they got them apart. He said they had to use way over 200#s of pressure to blow them out. When they came apart the pistons and bores were like new. The problem was the seals literally welded themselves. I think they charged me about $50 and it was well worth it.

I agree with Jose, they probably had the ability to use hydraulics on them. In addition, if you are going to soak them, use diesel fuel which is really kerosene and on;y a few$ a gallon. I would use a 5 gallon bucket, put the calipers in them and then cover them with the fuel. One warning, that stuff stinks so leave the bucket outside while they are soaking
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Re: M6 front brake caliper

Post by hornhospital » Thu Jan 14, 2021 5:08 am

Those contemplating using compressed air to force the pistons out, please take precautions to safely arrest the piston travel when it finally comes out. I have a friend that broke several fingers when a disc brake piston blew out of the caliper. He figured he'd just catch it before it hit the floor. Needless to say, he dropped it anyway. #-o
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Re: M6 front brake caliper

Post by Ralph in Socal » Thu Jan 14, 2021 5:50 am

As Jose said, use a grease gun. It works and the only drawback is having to clean up the mess. I’ve done it before and it’s much safer.

When using air, set a 6 inch long 1x2 in the pad slot for the piston to stop against. Prevents the piston from popping all the way out so you can maintain pressure and push the other pistons out.

Have a rag over the work and where you’re injecting air as brake fluid can spray out. You shouldn’t have to ask me why I suggest the rag

Typical issue is buildup and rust that locks the piston in the bore. The grease gun works best unless you set up a hydraulic rig with a pedal set and master cylinder. A clutch master would probably work well


BTW - it is not recommended to separate the caliper halves.
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