Caliper Piston Gasket Expanding

[alvocado]

I noticed during my last brake service that the piston gasket was expanded on one caliper and I had a difficult time pressing it back in. It popped out several times before I got the piston compressed and the caliper back in place. Would this indicate a bad o-ring on the caliper piston itself, possibly causing some fluid to reach the rubber gasket? The brakes are working fine with no pulling, etc. after a year but I want to address any issues before incurring a major brake problem. If the caliper does need service, what's involved with rebuilding it?

[pm26]

I noticed during my last brake service that the piston gasket was expanded on one caliper and I had a difficult time pressing it back in. It popped out several times before I got the piston compressed and the caliper back in place. Would this indicate a bad o-ring on the caliper piston itself, possibly causing some fluid to reach the rubber gasket? The brakes are working fine with no pulling, etc. after a year but I want to address any issues before incurring a major brake problem. If the caliper does need service, what's involved with rebuilding it?

 

On a typical brake caliper, there is an outer dust boot, and inner seal that fits into the groove in the cylinder bore. What you saw was obviously the outer dust boot. It normally does not pop out unless it is torn or was improperly installed initially. There are several versions of these dust boots and some are an absolute PITA to install - almost impossible without a special tool.

 

A torn or damaged outer dust boot is not an emergency and does not mean the caliper will fail - the inner seal would have to fall apart for that to happen. However, the dust boot protects the inner seal from grit and dirt, so it should be replaced.

 

Rebuilding a single piston caliper is usually simple, unless it is one with the built in parking brake mechanism, which is much more difficult to rebuild properly. On a front caliper, you need to remove the caliper from the truck, unscrew the brake hose and use a proper fitting bolt to plug the opening. Then remove the bleeder screw, remove the pads and place a piece of 2X4 between the caliper piston and the bracket. Then use a soft tip blowgun and compressed air (about 50 psi) to pop the piston out. Keep you fingers away as the piston will come out w th great force. After removing the piston, clean the piston with brake fluid. If there are stubborn deposits on it, very gently clean with wet 1000 grit sandpaper - but gently so you do not remove the chrome plating on the piston. The cylinder bore can be roughened up some with a 400 grit sandpaper or emery cloth. Then you need to use a small wire brush on a drill to remove the corrosion on the outer dust boot groove. If the caliper and its piston has a lot of rust on it, you are better off simply getting a rebuilt caliper. Usually if there are no pits or rust on the piston, I will rebuild the caliper. Clean the inside of the bore and the piston with clean brake fluid. Install the inner seal. Then install the outer boot and the piston. Do not force the piston in - it should slide in by hand.

[pm26]

Delete - duplicate post

[alvocado]

Thanks for the reply. I inspected the outer dust boot for tears during the brake job but didn't notice any so I'm assuming it's either defective or has weekend such that it won't hold it's shape. This is a front caliper so it should be straight forward to replace based on your instructions.

[alvocado]

One more question: any chance someone has the bolt size for plugging the fluid line?

[pm26]

One more question: any chance someone has the bolt size for plugging the fluid line?

 

On second thought, leave the bleeder screw in a and tight and use the brake hose threaded port to pressurize the caliper with compressed air to force the piston out. It works just fine. This way you do not need the bolt.