My shop is in an over-sized two-car garage attached to my home. My work is a calling, in the theological sense of the word. Ethical and moral questions arise every day – they are hard, sharp, unambiguous and unavoidable.

Rehabilitating disc brake calipers

Posted: December 27th, 2011 | Author: Administrator | Filed under: Motorcycle Repair, Wheels, Tires, & Brakes | No Comments »

K,

The pitting is due to accumulated water in the hygroscopic DOT 3/4 brake fluid finding its way through the chrome plating on the surface of the pistons to the steel of the piston itself and rusting it. The aluminum alloy of the caliper body is unaffected.

I have found that a piston with mild pitting can be re-used. If you can send me a few pix of your pistons, I can tell you whether I would re-use or replace them.

The most common problem with old/neglected disc brakes is drag – the pistons not retracting fully when the lever is released. I almost never see fluid leaking past the piston due to seal failure around the piston. The drag is caused by crud building up behind the sealing ring and forcing it too tightly against the piston. I remove the seal and clean out the crud from the seal groove with a dental pick or similar tool. Also clean any crud stuck to the back of the seal. Nine times out of ten, this restores the brake to good working order – with no need for stupidly expensive parts – e.g. $30 for a rebuild kit which is just the rubber o-ring/seal.

If the pistons are stuck in their bores, you can remove the caliper from the disc but keep all the hoses hooked up and use the lever to pump the pistons out of their bores. Alternatively, a grease gun fits the tip of the bleeder screw like it was made for it. Blank off the brake line hole with a short bolt and crack the bleeder screw a 1/2 turn or so. I have yet to fail to move a stuck piston with this method. If you have a caliper with more than one piston, you will have to devise an ad-hoc method to hold one in place while the other is removed. A small C-clamp often works.

All of this can be prevented if the brake fluid is changed once in a while.

Cheers,

Chris



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