How to Rebuild a Brake Caliper ?

It's a beautiful spring day, and a perfect time to break the winter seal on the garage and drive around in your classic Corvette, vintage motorcycle or whatever pride and joy you happen to have stashed inside. Off with the cover, and the engine turns over willingly, thank goodness. A quick warmup and you're off for a loop around town.



All of which is fine, until the engine seems a little ... reluctant to maintain speed. As you turn a corner, you catch that unmistakable whiff of burning sweat socks--you've got a dragging brake caliper. Sure enough,

one of the front wheels is too hot to touch. You limp the few blocks home at low speed. Now your Saturday morning drive has just mutated from a fun-filled jaunt into an afternoon's worth of work chasing down a replacement caliper. A quick trip to the auto parts store, followed by a round of phone calls to dealerships and stores within an increasingly large radius turns up nothing. You'll have to rebuild the caliper yourself.

Most brake calipers that fail get replaced with either a new part or a rebuilt one. Indeed, we picked up a rebuilt caliper for this story at the local NAPA store. Industry-wide, it's common practice to return used calipers to a central rebuilding station where the caliper assembly is rebuilt. It will be stripped to the basic casting, cleaned of all vestiges of road grime by automated equipment, and glass-bead-blasted or shot-peened to remove any and all corrosion and rust. After a quick inspection, it's reassembled with a new piston, seal and bleeder bolt. Inside the box with the like-new caliper assembly, you'll find fresh sliding pins and any other hardware that should be replaced--and sometimes, new pads as well. All you need to do is install the caliper, bleed it, and you're back on the road in a few minutes. Often, the modest price of this rebuilt assembly is less than the over-the-counter price of the aggregate parts you'd have to buy separately to rebuild the unit yourself. No wonder pro mechanics rarely bother to rebuild calipers anymore.

But you've gotten the bad news from at least a half-dozen countermen in the last hour--no rebuilt, or even new, calipers until the middle of next week. And that scotches your plan for traveling to tomorrow afternoon's car show. One savvy counterman, an old-school guy, has a suggestion: He knows that the cylinder bore on your caliper is common to several other cars. And he does have generic brake piston seals in your size. You rush over to his store and pick up a set of seals and a couple of pints of fresh brake fluid. Price: less than your lunch at the fast-food drive-through window.

One caveat: If your frozen caliper is a rear caliper, with attendant parking-brake cables and internal antipushback hardware, you'll need to consult the service manual for rebuild advice. Also, take note that we're talking about calipers here. If your rotors are out of true, worn too thin or scored by the dragging pads, you'll need to remove and machine or replace them as well. Your brake system is not a good place to cut corners. Okay, I'm off my safety soapbox.

Brake It Down

First loosen the lug nuts, block the rear wheels and get the car up on jackstands. Here's your chance to break all of the caliper bolts loose while that caliper is still mounted to something solid. Judicious tapping and some penetrating oil should help break the bleeder bolt loose. Use a flare-nut wrench on the bleeder--it's less likely to round off the small, hollow bolt.

Odds are, if the rubber flex line screws directly into the caliper casting, you may need to remove the rubber brake line at the far end first. Ditto when using a flare-nut wrench on the brake line. Consider replacing the steel or rubber brake lines if they are older than your dog.

On the Bench

Strip the caliper of the pads, shims, springs and sliders or pins. Remove the outer dust seal, too. Now you need to remove the piston--but you'll quickly realize there's no way to grab it. Get a piece of wood slightly thicker than both brake pads stacked together. Put that wood in the caliper aft of the piston, and add low-pressure compressed air to the inlet port. Pop! Your piston is out. And so is your little finger if you're not paying attention. The same goes for your corneas if you don't cover the whole deal with a shop towel to contain the flying brake fluid. If the piston is stuck--which is how we got into this mess--the process may take a little more time and pressure. If your caliper has more than one piston, use more wood, clamps or whatever to release one at a time.

Left: Use some nonscratching tool, like a popsicle stick or this nylon cable tie, to pry the seal loose from its groove. Right: We sawed this GM caliper in half so you could see inside--this shows evidence of serious corrosion pitting in the base of the bore. We cleaned it with 600-grit paper.

Inspect the piston. Rebuild kits usually include a new piston, but if you're gleaning from generic parts, you may be stuck, literally. Minor surface blemishes can be polished out with wet 2000-grit sandpaper, but any pits in the metal means replacement will be necessary. I've had replacement pistons machined out of stainless-steel bar stock for oddball cars.

To get the O-ring free, use something that won't damage the caliper bore. Odds are, corrosion built up between the piston and bore is what made things stick. Unlike surface damage to the piston, minor pitting of the bore is no big deal--the square-cut O-ring seals between its recessed groove and the piston, not to the outer bore. Clean up the bore surface with wet 600-grit wet-or-dry paper, then thoroughly clean the entire caliper of all dirt, rust, abrasives and old brake fluid with hot, soapy water. Some two-piston calipers may have internal rubber seals--so no hydrocarbon solvents, please.

All clean and dry? Lubricate the piston with clean brake fluid and slide it into the bore. Once the piston is squared up, it should slide in all the way with moderate finger pressure. If not, find out why. Remove the piston again, install the new seal, lubricate again with fresh fluid and push it on home. Add the outer dust boot, and reinstall the freshly rebuilt caliper. Repeat on the other side. Don't be lazy and rebuild or replace only one side. Brakes should always be maintained as an axle set, not one corner at a time. Top off the reservoir with fresh fluid, and bleed the system (see "Bleeding Your Brakes," April '07).

Once everything is clean, put it all back together. Be sure to tidy up or replace any sliding pins or raceways so the caliper can move back and forth as the pads wear.

Brake Pad Tech

Not all brake pads are created equal.There's a choice today of various friction materials used in pads to do the work of actually stopping your car. Asbestos used to be a major component of brake pads, but OSHA regulations now make this carcinogen scarce. Organic materials such as Kevlar, often found in OEM brake applications, represent a compromise of pad life, braking aggressiveness, dusty wheels and noise. If you're interested in a longer-lasting pad material, look into a premium metallic or ceramic lining. These materials can sometimes squeal or hiss, wear out the discs faster and cover the front wheels with ugly dust. Performance cars, like that Corvette of yours, might stop better with metallic pads. The downside is that those polished rims--aside from the cost--will be covered in gray dust after a day's drive.