Instructables
Picture of Servicing Front Brake Calipers
Brake lights in front of you flash suddenly, and for no discernible reason. You mash the brake pedal to avoid rear-ending a driver who must have suddenly remembered leaving the stove on. Your car stops, but yoru’ve swerved well into the adjacent lane, where, fortunately, an alert driver slowed and pulled over to give you some clearance. Too close. Your panic braking attempt should have had two different results: Your car should have slowed more rapidly, and should have tracked in a straight line as it slowed. There’s obviously something wrong.

Now what?  This Instructable will help you service faulty front break calipers. Try these repairs yourself, save money, and stay safe!  

This project was originally published in the January 2001 issue of Popular Mechanics.  You can find more great projects at Popular Mechanics DIY Central.
 
 
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Step 1: Check it Out

Your car pulled to the left, but the problem is more likely to be at the right front wheel. It didn’t brake as well as the left, so the car pulled in the opposite direction. 

Remove the wheel and take a look. Don’t be surprised if you see that both shoes have a reasonable amount of brake lining left. The problem is more likely to be a bad caliper, that big hydraulic clamp that forces the brake shoes against the disc-brake rotor. Most cars have sliding calipers with a single hydraulic piston. When you step on the brake pedal and that piston is pushed out, it forces the inner brake shoe against the inner side of the rotor. The caliper simultaneously slides inward, pulling the brake shoe in the outer side of the caliper against the outer surface of the rotor. Some cars have fixed calipers with one or two pistons in each side of the caliper (two or four pistons total). When you step on the pedal, all of the pistons force the brake shoes against the rotor. 

Check the brake fluid level in the reservoir and if it’s been topped up to maximum level, siphon out a small amount (otherwise it may overflow during the test). Set up the clamp so the top edge of the C is against the midpoint of the inboard side of the caliper (behind the piston) with the tip of the forcing screw directly opposite. Depending on the shape of the caliper, the forcing screw could also bear against the back side of the outboard brake shoe or even against the the  rotor surface. 

Turn the forcing screw and the caliper should move smoothly as the piston is pushed back. 

If the caliper passes this test, make one more: Check the bleeder valve to be sure it loosens. If it doesn’t, you can drill it out and install a replacement. But this can be a difficult operation, and installing a remanufactured caliper is a safer bet. If the caliper binds or moves only with unusual effort, there are two possibilities: The caliper piston is frozen in place (the most common), or the caliper is not able to slide because of corroded bolts (along which most calipers slide), cocked or damaged bushings through which the bolts go, or rusted sliding "ways" (guides) in the anchor that holds the caliper.  You'll have to remove the caliper to see.