4 Reasons Your Caliper Piston Won’t Compress and What to Do

Replacing the brake pads is generally not difficult, but if the caliper piston is seized, it will drastically aggravate the situation. Here's why it happens

Caliper Piston Won’t Compress

Whether you are running into trouble changing your car’s brakes, or the vehicle is pulling to one side and wearing through brake pads, you might be dealing with a caliper piston that won’t compress. What causes this situation, and what can you do about it?

In this guide, we look closer at what’s causing the seized caliper pistons. We also show you a few methods to compress the troublesome caliper piston. Finally, we examine the cost of replacing a seized caliper. 

Causes of Brake Caliper Piston Won’t Compress

When the brake caliper piston won’t compress, it’s likely due to a rusty or corroded caliper piston, caused by a bad piston boot. If it’s the rear caliper piston, consider the chance that the incorrect wind back procedure is being used. 

Here is a more detailed list of why your brake caliper piston won’t compress:

1. Corrosion

Rusty Caliper Piston

The most common reason why the caliper piston does not move is that the caliper piston is rusty or corroded due to a damaged caliper piston boot. Brake calipers can’t operate properly if corrosion or rust is involved. Any level of rust can affect the ability of the caliper pistons to compress.

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

RELATED: Brake Caliper Sticking (Causes & How to Prevent it)

2. Worn Boot or Seals

Damaged Caliper Piston Boot

Over time, the piston seals become worn. Eventually, the seals will fail completely, causing the piston to remain seized. 

It could also have to do with the rubber boot on the piston that protects it and keeps it lubricated. As the rubber tears or gets damaged, water will enter the piston and it will cause it to seize up. 

3. Collapsed Brake Hose

A common cause of a stuck brake caliper stems from the hose. If the hose starts to break or crack, brake fluid flows onto the piston. Because it can’t get back to the master cylinder where it belongs, the caliper is going to stick.

However, there are times when the caliper itself isn’t actually stuck. In some instances, the broken hose situation just causes the brakes to feel the same way. Do an inspection to discover where the problem lies.

4. Incorrect Wind Back Procedure

If the problem occurs with the rear caliper, it’s possible that you aren’t following the guidelines of the service procedure. Remember that rear calipers operate differently from the front. You must use the right steps to get them compressed.

You can’t simply push rear calipers back in. They must first be turned and pushed at the same time. You can use a wind-back tool to operate the rear caliper pistons with ease. 

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 Caliper Pistons

1. C-Clamp

Position the C-clamp over the caliper and up to the end of the piston. There’s a screw on top that must be adjusted correctly. You will want to use wood to shield the piston surface from damage. 

As you turn the screw, the pressure increases, allowing the piston to be compressed. Tighten this until the caliper is compressed where you need it. 

2. Channel Locks

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

Squeeze the channel locks together, so the piston is also compressed. You will want to start gently and increase the pressure as needed. 

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 to have the push bar hanging out.

Put the caliper compressor alongside the caliper in front of the pad. Pump the compressor tool down while the push bar applies pressure to the brake pad. This action should lead the pad to push into the piston so it can retract. 

4. Screwdriver

You often don’t need any fancy tools to compress a caliper piston. In fact, your standard screwdriver might be able to finish the job nicely. 

Put your screwdriver into the caliper window. You must hold onto it tightly as you fit it in the openings. The screwdriver must dip until there’s an equal amount of balance of force. The pulling motion should cause the brake piston to pull back a little.

When the screwdriver is in place, pull back slightly. You want the pressure to be firm against the pad in an outward motion. You can now put your screwdriver between the piston and brake pad. From here, you should be able to push the piston back with a gentle motion. 

Cost to Fix Seized Caliper

If you can get the caliper compressed with one of our tips, you might not need to replace it. Each situation is different. In fact, the problem could be the brake hose causing the same symptoms, so the caliper piston might actually be in working condition.

However, if you need to replace the brake caliper, you might spend between $250 and $500. The parts cost about $100 to $300, while the labor might end up being between $150 and $200. The cost depends on the type of vehicle you drive and your local labor rates. If you can replace the brake caliper yourself, you can save some money on the labor, but the majority of this expense falls back on the part itself. 

You can sometimes restore the caliper with a new piston and seals, but it’s often cheaper and easier to buy a new caliper instead.

Magnus Sellén
Written by:

Magnus is the owner and main author of Mechanicbase. He has been working as a car mechanic for over 10 years, and the majority of them specialized in advanced car diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.

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