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Caliper Piston Won’t Compress: Common Causes (and What to Do)

Whether you’re having trouble changing your car’s brakes or your vehicle is pulling to one side and wearing out your brake pads, you may be dealing with a caliper piston that won’t compress. What causes this situation and what can be done about it?

When the caliper piston does not compress, it is most likely due to a rusted or corroded caliper piston, caused by poor piston bellows. If it is the rear caliper piston, consider the possibility that the incorrect rewind procedure is being used.

In this guide, we take a closer look at the cause of seized caliper pistons. We also show you some methods to compress the problematic caliper piston. Finally, we look at the cost of replacing a seized caliper.

Seized caliper piston Causes

1. Corrosion

The most common reason for the caliper piston not moving is that the caliper piston is rusted or corroded due to a damaged caliper piston boot. Brake calipers cannot function properly if they become corroded or rusted. Any level of rust can affect the compression capacity of the caliper pistons.

If you use your vehicle regularly, this probably won’t happen to you. However, inactivity and driving on rainy or salty roads can often lead to corrosion, if the trunk is slightly damaged.

RELATED: Stuck Brake Caliper (Causes and How to Prevent It)

2. Worn boots or seals

Over time, piston seals wear out. Eventually, the seals will fail completely, causing the piston to stick.

It could also be related to the rubber boot on the piston that protects it and keeps it lubricated. As the rubber breaks or deteriorates, water enters the piston and jams it.

3. Collapsed brake hose

A common cause of a stuck brake caliper comes from the hose. If the hose begins to break or crack, brake fluid is leaking onto the piston. Because it cannot return to the master cylinder where it belongs, the caliper will get stuck.

However, there are times when the caliper itself is not actually stuck. In some cases, the broken hose situation causes the brakes to feel the same. Perform an inspection to find out where the problem is.

4. Incorrect rewind procedure

If the problem is with the rear caliper, you may not be following the service procedure guidelines. Remember that the rear calipers work differently than the front ones. You must use the correct steps to compress them.

You can’t just push the clamps back. They must first be turned and pushed at the same time. You can use a rewind tool to easily operate the rear caliper pistons.

If you have an electric parking brake, you may need a diagnostic tool to rewind the parking brake.

RELATED: How to Change Brake Pads with an Electric Parking Brake

How to Compress Stuck Bore Pistons

1. C-clamp

Place the C-clamp over the caliper and to the end of the piston. There is a screw at the top that should fit properly. You will want to use wood to protect the surface of the piston from damage.

As the screw turns, the pressure increases, allowing the piston to be compressed. Tighten it until the clamp is compressed where you need it.

2. Channel blocking

With the channel locks, you can perform similar steps as the C-clamp. Place the channel locks where one end is behind the footboard and the other end is behind the pad. You want the caliper and brake pad to be located between the grips.

Tighten the channel locks so the plunger also compresses. You will need to start slowly and increase the pressure as necessary.

RELATED: Symptoms of a Bad Brake Caliper

3. Piston Compression Tool

Brake caliper compression

If you have a special compression tool to work with the pistons, your job will be easier. Start by making sure the compressor is properly adjusted for the job. You don’t want the push bar to drag.

Place the caliper compressor next to the caliper in front of the pad. Pump the compression tool down while the push rod applies pressure to the brake pad. This action should cause the tampon to push against the plunger so it can retract.

4. Screwdriver

Often, you don’t need fancy tools to compress a bore piston. In fact, your standard screwdriver could finish the job.

Put your screwdriver in the caliper window. You should hold it firmly when placing it in the openings. The screwdriver should be immersed until there is an even balance of force. The pulling motion should push the brake piston back slightly.

When the screwdriver is in place, pull back slightly. You want firm pressure against the pad in an outward motion. Now you can place your screwdriver between the piston and the brake pad. From there, you should be able to push the plunger back in one smooth motion.

Cost to repair a seized caliper

If you can compress the collet with one of our tips, you may not need to replace it. Every situation is different. In fact, the problem could be that the brake hose is causing the same symptoms, so the caliper piston could be working.

However, if you need to replace the brake caliper, you could spend between $250 and $500. Parts cost between $100 and $300, while labor can cost between $150 and $200. The cost depends on the type of vehicle you drive and local labor rates. If you can replace the brake caliper yourself, you can save money on labor, but most of that expense goes toward the part itself.

Sometimes you can refinish the caliper with a new piston and gaskets, but it is often cheaper and easier to buy a new caliper.


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