by Dave Zeckhausen
What is caliper rebuilding?
Caliper rebuilding is the replacement of worn or damaged parts. Typically it involves replacement of pressure seals and dust boots that have become brittle from age and/or excessive heat from heavy use, such as racing. Occasionally, pistons may need to be replaced if they have become corroded, pitted, dented, or even melted.
How often should calipers be rebuilt?
There is no hard and fast rule for caliper rebuilding frequency. Some calipers won't need to be rebuilt during the life of the vehicle. High-powered race cars may need to have calipers rebuilt several times per season. At each pad change, inspect the calipers carefully. Any of the following conditions call for a caliper rebuild:
Dust boots are vaporized, cracked, or simply no longer soft and pliable
Brake pads are dragging, causing them to overheat
Pistons are difficult or impossible to retract
Brake fluid is leaking from around one or more pistons
Calipers are being refinished (i.e., stripped and painted or powdercoated)
How do I know what rebuild parts to purchase?
It's critical to determine the piston sizes in your caliper. Many calipers use staggered pistons, in order to hold the pads parallel to the rotor during braking and ensure even pad wear. Do not measure one piston and assume all the others are the same size.
For StopTech calipers, see: How to Measure StopTech Pistons.
For all other calipers, measure piston diameter directly.
Remove brake pads.
Disconnect brake lines and unbolt calipers from vehicle.
Drain fluid from calipers. Remove brake line fittings from the calipers. With the brake line inlet port held over a container to catch fluid, press the pistons until they are retracted fully. A race caliper spreader tool, like the Girodisc unit shown below, can make short work of this by compressing all pistons simultaneously.
Clean calipers so brake fluid doesn't damage finish. Plug the caliper inlet with a rubber stopper or bolt and wash the caliper with gentle car wash soap and water. Dry with a soft microfiber towel and/or compressed air.
Do not use strong solvents, such as mineral spirits or “Brake Cleaner." These can damage painted, powdercoated, or anodized finishes. Treat the finish on your calipers the same as you would treat the paint on your car.
Extend pistons. Place a piece of wood or a brake pad into the caliper to prevent any individual piston from ejecting completely. Use an air gun with a rubber tip to apply a jet of compressed air to the inlet until all pistons have extended far enough to be pulled out by hand. Once any individual piston pops out, any additional compressed air is going to flow from that opening and the remaining pistons won't move. Keep fingers clear, as a piston may eject suddenly and with force. Use eye protection, in case brake fluid sprays when compressed air is used.
Remove each piston carefully from its bore by hand, keeping it parallel to the bore until free, so that neither piston nor bore is scratched. Any scratch or dent on the sides of the piston will render it unserviceable, and it must be replaced. Otherwise brake fluid will leak past the pressure seal when the piston flaw passes in front of it.
Remove dust boots by grasping the flexible portion with thumb and forefinger of both hands, applying tension as evenly and over as wide an area as possible. If that doesn't work, a small flat blade screwdriver may be used to pry them out carefully.
Remove pressure seals, taking care not to damage the piston bore or seal groove in the caliper body. A thin, flat spatula, a 90-degree bent stylus, or a mechanic's pick works well.
Clean pistons, bores and seal grooves and inspect for scratches, dents, corrosion or other damage.
Install new pressure seals. Inspect each seal to ensure it is clean. Lubricate seals with brake caliper ASSEMBLY fluid. (NOTE: This is NOT the same as brake caliper lube, which is used to lubricate sliding, metal parts. Using caliper lube, instead of caliper assembly fluid, may damage your seals and contaminate your brake fluid, leading to failures of ABS and stability control parts. Make sure you use the right stuff!) Insert pressure seals by hand, into the seal groove for its piston bore, making sure the correct size seal is used for each. Check carefully to make sure they aren't twisted, by running your bare finger across the surface. If it is twisted, remove and reinstall.
Install dust boots onto each piston, prior to installing piston in the caliper. Make sure you do not get any lube on the dust boots. If you do accidentally get lube on the boots, clean them off with soapy water before continuing. Otherwise the boots may refuse to stay seated and will pop out later.
Install pistons. Lightly coat the bottom and sides of the pistons with Brake Caliper Assembly Lube. Slide the pistons halfway into the bores. Rock or rotate, if necessary, to ensure they are inserted straight, so as not to damage the piston or seal.
Apply even pressure to slide each piston further into the bore. The pistons should go all the way to the bottom of the bore with hand force. If this is not possible, STOP! Remove the piston and inspect the seal to see why it is getting hung up. Debris in the caliper's pressure seal groove may be pushing the new seal into the path of the piston. If you force the piston into place, it will tear a chunk off the pressure seal and the caliper will leak.
Press dust boots into place carefully, ensuring the outer metal ring, molded into the boot, is installed nearly level with the flat area surrounding it. If the boot is pressed unevenly, it is possible to damage the silicone or rubber material inside the metal ring. It may not be possible to push the boots in place with your fingers, in which case a curved piece of wood, plastic or metal may be used. Do not let brake fluid come in contact with the boots, as this will soften them and make them easy to tear.
Reinstall caliper on the vehicle, using new crush washers for the banjo fitting (if applicable) at the caliper brake line inlet. Reinstall pads.
Bleed brakes with fresh fluid and fill the master cylinder reservoir.
Test brake system for leaks. Apply pressure to the brake pedal for several minutes, inspecting each caliper for any sign of leakage.