Calipers, Cylinders & Hydraulics

The caliper has stopped floating. On a sliding caliper the body must move along its guide pins so the outer pad is pulled onto the disc, and when a pin seizes in its bore the piston pushes the inner pad hard while the outer barely touches. The cause is nearly always a split boot letting water reach the pin. Fit new pins, boots and the correct high temperature grease at the same time as the pads. Doing the pads alone leaves the new inner pad to wear out just as fast.

That is the most likely explanation, though check the hose first. A caliper whose piston has corroded in its bore, or whose seal has swollen with contaminated fluid, holds pressure on the pad after you lift off, and the wheel heats up, the pad glazes and the car pulls. A collapsed flexible hose produces exactly the same symptom by trapping pressure. Crack the bleed nipple with the wheel hot: if the pad releases immediately, the hose is holding pressure rather than the caliper.

Rebuilding makes sense when the piston and the bore are clean and the fault is a perished seal or a split boot. It makes no sense once the bore has pitted, because a rebuilt caliper on a corroded bore will drag again within a season and you will have paid twice. Inspect the bore with the piston out. On rear calipers carrying an electric parking brake mechanism, the internal gearbox complicates rebuilding further, and replacing the sub assembly is usually the honest answer.

Work through four possibilities in order. If it is a rear caliper with an electric parking brake, the motor must be retracted through a scan tool in service mode and no amount of force will substitute. If it is a mechanical rear caliper with a cable, the piston screws in rather than pressing. If neither applies and the piston still resists, either the flexible hose is trapping pressure, or the piston has corroded in its bore and the caliper needs replacing. Never lever against a disc.

There is no absolute rule, and there is a practical argument for it. Calipers on the same axle have done identical work in identical conditions, so when one seizes the other is usually not far behind, and braking balance depends on both sides applying and releasing together. Where budget allows, replacing across an axle removes the doubt. At minimum, strip, clean and regrease the guide pins on the opposite side while the car is in the air and the tools are out.

Guide pins, boots and small hardware sit right at the bottom of the category, which is fortunate because they cure a large share of complaints. Caliper brackets are modest. Complete caliper sub assemblies cost the most, particularly rear units containing an electric parking brake motor and gearbox. The live minimum and maximum are shown in the band at the top of this page. Diagnose before ordering, since the difference between a set of pins and a rear caliper is considerable.

Read the listing carefully, since the terms are not interchangeable. A caliper body sub assembly is the housing with its piston and seals but usually without the carrier bracket, pins and hardware. A complete caliper sub assembly is what most people mean by a caliper: the body plus the parts needed to mount it. Which you need depends on whether your carrier and pins are serviceable. If the bracket is sound and the pins are new, the body alone can be the cheaper route.

Two innocent explanations and one serious one. As pads wear, the pistons sit further out and the fluid needed to fill behind them comes from the reservoir, so a slow fall over months is normal and reverses when new pads push the pistons back. A sudden fall is a leak, and it will be at a caliper, a hose, a line or the master cylinder. Look for damp patches at each wheel and behind the reservoir. Never simply top up and carry on without finding the cause.

With care, yes. Support the vehicle properly, clamp or plug the hose before disconnecting, use a flare spanner on the union, fit new copper washers on any banjo connection, torque the carrier and slide bolts to specification, then bleed thoroughly and test at walking pace before going near a road. Rear calipers with an electric parking brake add a scan tool step at both ends of the job, to retract the motor and to reset it. If that tooling is unavailable, use a workshop.

A dedicated high temperature synthetic caliper grease that is compatible with rubber, since ordinary lithium greases swell the boots and petroleum based products destroy them. Apply a thin film to the pin, not a packed bore, or hydraulic lock will stop the caliper sliding home. Grease also belongs on the pad abutment points in the carrier and on the back of the backing plate where the shims contact, and never anywhere near the friction face or the disc.

Not reliably. Caliper size follows disc diameter and vehicle weight, so a single model may carry two or three caliper specifications with different piston diameters, and fitting a mismatched caliper changes the front to rear braking balance in a way that the stability system will notice. The mounting pattern often matches, which is what makes the mistake possible. Match the OE number from the old unit, and where the reference brings back nothing, send it through the quote form.

Yes. Send the number through the quote form and a figure comes back against that side of that axle. Calipers are handed, so the reply is only useful if the reference you supply is the one from the corner you are working on. Electric parking brake calipers are the ones most often quoted rather than priced, since the motor variant differs across trims. While you are asking, include the guide pins, the piston seal and the dust boot. A caliper body in good condition with seized pins is a repair rather than a replacement, and the seal kit costs a fraction of the assembly.