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caliper piston rebuild procedure

Discussion in 'E36 M3 (1995-1999)' started by tjrinaldi, Mar 21, 2011.

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    tjrinaldi

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    After searching, I can't believe no one has asked about this before...

    I am currently in process of rebuilding my ///M calipers for E36, and am doing so because of a stuck/rusted piston. I am under the impression that the rebuild is cleaning everything up and re-installing new seals (from the kit). IS THERE ANY NEED FOR LUBRICANT/SEALING COMPOUND/ETC AND WHAT IS IT? It is entirely possible I am missing something huge here, but I don't know.

    If there's anyone out there who knows about this please let me know if a special compound is needed in there and what kind, perhaps, they recommend. Thanks in advance.....hopefully someone knows about this before I re-install the new seals...
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    MGarrison

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    No lubricant needed, as far as I know. Once filled, I suppose the brake fluid _may_ act as any lubricant. If the piston or the piston bore in the caliper is particularly rusty, you may be better off to replace the caliper than attempt to use it. Last thing you'd want is rust damage compromising the tight clearance/tolerances which normally doesn't allow for any brake fluid leakage past the piston. I don't know the exact spec, but I think the pressures involved when the brakes are applied is hundreds of psi, or between 1 & 2 thousand psi. A pitted piston or caliper bore might leak under brake system pressure, or damage the piston seal and allow for leakage.

    If the piston and caliper's piston bore are clean, I think you should be able to press the piston back in without too much resistance.

    cwbiii guest

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    Typically you may need to hone the cylinder... one is on the tool wall or tool carousel in many auto parts stores.(NAPA) Its relatively simple to use and comes with directions.
    Similar to a hone for engine cylinders, only smaller.

    Chuck
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    granthr

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    Before reassembly coat everything lightly with brake fluid, it will make it go back together easier, but not easy.

    Be careful when removing the piston. The easiest and most dangerous way is with compressed air. Shove lots of towels between the piston and the arm that holds the brake pad. You don't want that thing to go flying. Goggles and a face mask are also a good idea, as you will be spraying brake fluid out as well.

    Clean all the exterior surfaces with a wire brush, being careful not to score or damage the machined surfaces. Clean up the machined surfaces with a lint free cloth and brake fluid. If there is any damage to the machined surfaces the caliper might not be salavable.

    Once apart and cleaned up look for pitting on the piston and cylinder bore. If there is pitting along the sides you probably should get a whole new caliper. If there is pitting on the bottom (where there is no lateral movement between the two pieces) it might be okay to reuse.

    Rebuild the caliper with all new seals, boots, and lock rings if it has them. This is not a super difficult job, just be sure you take your time and not to damage anything.
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    MGarrison

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    Just wanted to reiterate - wire brush on EXTERIOR parts only!

    It takes very little air pressure to pop out the piston (easily done with a rubber-tipped blower-gun: http://www.harborfreight.com/rubber-tip-air-blow-gun-3962.html).

    If you push the air gun into the bleeder screw hole hard and give it a full blast, you'll shoot the piston out like firing a gun (overkill, potentially messy & unsafe). A couple light spurts of air should be adequate.
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    CRKrieger

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    Ummm ... guys, if you put a piece of wood into the caliper where the rotor and pads would normally be, the piston won't go flying anywhere.

    I don't sweat a little pitting on the piston or walls if it will clean up with a light sanding (like 1000 grit or higher cloth). I've used hones before, but not always. The piston-to-seal surface doesn't see significant movement and the piston seal will conform to minor imperfections quite easily. The biggest enemy in your caliper is a damaged boot that allows dirt and moisture past it. Otherwise, things pretty much stay the same inside there for tens of thousands of miles.
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    MGarrison

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    Yes, but that requires advance planning and having wood on hand... all of which has gone looonng by-the-wayside by 3:45 am when you realize you _still_ have to rebuild the brake calipers before being able to drive to work in the morning! :p
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    CRKrieger

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    OK; yeah. Been there; done that. Actually took apart a seized front caliper, hand-sanded the corrosion out of it and slapped it back together without a boot (the old one was torn all to hell and I didn't have a rebuild kit on hand), put it back on and bled it. It ran that way for about a week until I got a rebuilt one to replace it. One of those times that understanding the simple technology tells you how much you can get away with. ;)

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