What causes a caliper to stick?
 

I think my front right caliper is sticking because the car pulls a bit, and that wheel seems hotter after a driving. I plan to rebuild both my front calipers (rears to follow), but I don't understand what causes a caliper to stick. I have never rebuilt a caliper before, but have done brake masters and clutch masters and slaves in other cars. This previous experience, however, has only taught me that o-rings leak. That knowledge does not explain why a caliper would stick. (It would explain why you might lose break pressure).

So, what causes a caliper to stick? Does a rebuild (which I understand is replacing the seals) fix the problem?

Thanks for any insight,
Doug

yea doug. i would add new rubber brake lines too. because they can collapse, and swell, and do all sorts of hideous things to make your brake bind. your bleeding anyways, and since you are there you need to take off one side of the line, may as well change the rubbers. i think the pistons can get corroded and stuff too. you will be amazed at the amount of crap in your calipers.

cliff

The 2 main causes are corrosion of the piston walls because fluid wasn't changed regularly and internally swollen rubber lines from the same lack of maintainance(age is a factor in the deterioration of all rubber parts as well)

brake fluid needs to be chaged annually and it helps to exercise the caliper pistons at that time as well

My calipers had a small amount of corrosion between the piston seal and the dust boot. It was just enough to restrain the pistons from applying full force to the pads.

A good way to tell if they're dragging is with a pyrometer (lazer thermometer ~$90) on the rotors after a drive.
If you crack the bleed screw and fluid spurts on its own(residual pressure), the brake line may be bad or the MC may be sticking if it's both sides.

The caliper piston is sealed with a square-cut(?) o-ring that squashes and deforms and springs back to retract the piston. It's helped by the pad spring guides but there's not much there to overcome any resistance.

It's almost alway the front right caliper that sticks, mine stuck, and I've seen references to that one sticking several other times on this board, and even on the miata board once or twice since I started checking them out.

In my case it was crud that had built up on the piston and inside the bore of the caliper. It was a shiny, hard, brown varnish looking stuff. Probably because most people don't flush their brake fluid ever, and so unless you've had your car since new and done it yourself every year or two it's probably built up some crud inside the system.

In my car it would only pull if I got them extra hot, but that wheel was always a little warmer than the other. When I finally got around to it I rebuilt both fronts, replaced all 4 rubber lines, flushed and bled the system. Actually I don't know why I didn't rebuild all 4 calipers, but I should have. Seems silly to do just 2.

I would say most likely the brake lines. Flexible brake lines swell internally and restrict the flow of fluid.
You can generate a tremendous amount of pressure with your foot and that will make the pad press on the rotor. When you let your foot off, the line has too small of a hole to allow the fluid to flow back to the master cylinder. Brake lines are a cheap first fix at the problem.
If it's a 79 then most likely hasn't had the lines replaced and are probably due.
The piston should slide easily when it's off the car and sitting on the bench with no fluid pressure on it.
Rust or crud as others above mentioned might be the problem.

I had the same problem.

http://forums.pelicanparts.com/porsche-911-technical-forum/169522-left-rear-brake-problem.html