Why would a rebuilt caliper still leak?

[John Kat]

Whenever you pull apart a main or slave cylinder it is a good idea to clean it up with a 400 wet/dry paper, and use it wet.

This is because over time there will be small pits and/or grooves in the wall. They may not be deep, but they can allow a tiny amount of fluid to pass, and that will decrease brake line pressure, and decrease braking ability.

There are hones for this, as well, but they strip metal away faster than paper, and their overuse can enlarge the cylinder. If you do not have a micrometer, there is no way to know if a PO has done it, so the wet/dry paper is often best.

Work slowly and carefully, so you get even cleaning.

I'm not sure I agree with this
The surface that must be unmarked is the surface of the piston not the cylinder.
The piston moves against the seal but is only guided by the cylinder.
Many times a small pit of rust develops on the surface of the pistons and the leak will start
Having a nice and smooth cylinder is of course a good thing but if the piston's surface is not perfectly smooth you might aswell throw it away...

[TooManyToys]

As John said, if it's not a copper washer, then it would be the piston's surface or the area within the tangential groove that the square seal fit's into. The caliper bore per say does not seal anything.

[silverhorse47]

I'm not sure I agree with this
The surface that must be unmarked is the surface of the piston not the cylinder.
The piston moves against the seal but is only guided by the cylinder.
Many times a small pit of rust develops on the surface of the pistons and the leak will start
Having a nice and smooth cylinder is of course a good thing but if the piston's surface is not perfectly smooth you might aswell throw it away...

Good insight. But the rubber cup on the piston seals against the cylinder wall, so any grooves or pits in the cylinder will prevent the cup from sealing, correct?

S.

[Chris G]

Try using some red rubber grease, had a similar problem with a 4x4 caliper fitted new seals and pistons (aftermarket) but started leaking past the seal! stripped it again using a smear of red rubber grease on the pistons and seals never been a problem since.

[John Kat]

Good insight. But the rubber cup on the piston seals against the cylinder wall, so any grooves or pits in the cylinder will prevent the cup from sealing, correct?

S.

You are right but only in the groove where the rubber cup is located

[silverhorse47]

Nothing pleases me more than hearing that I can distinguish a groove from a cup, and a cylinder from a piston.

S.