After an incident that was entirely my fault, I obtained a caliper seals kit and a reconditioned sierra piston. Everything went fine transferring the piston internals across from the old one to the new one, the seal was easy to get in to the cylinder, I obtained the correct (red) grease and even managed to get the boot into the groove at the top even though the guy at the local motor factor said it was impossible (smug). I used my improvised rewind tool to get the piston all the way down then I filled the caliper via the bleed nipple using a syringe while I had it in the vice.

Once I get it back on the car though, it just pisses fluid from around the seal whenever I apply any pressure to the brake pedal. Is there some magic I'm not doing, or did I get sold a dud piston that has been ground down too far and won't make a good seal with the rubber ring in the cylinder?

Are the piston seals tapered?

Meaning that there is a write way and a wrong way to insert it. If this is the case, then this could explain why your seals won't.

Yes, I can see how that would cause a problem, but I don't think it's tapered, it really doesn't look like it is... does anyone know for sure?

Sounds like you have the seal in the wrong way around, the wide shoulder points in, it should look like an arrow head

[Edited on 22/7/12 by mark chandler]

More likely to be a problem with the seal at the back for the handbrake mechanism IMHO.

As you obviously are not sure about what you are doing, with a safety related item such as brake calipers I strongly feel that you should just get exchange calipers and not try to do this yourself.

Just my 2p worth

Cheers

Mark