Brake jobs aren’t so bad, but when you get a piston that just won’t move a simple job can turn into a nightmare. Don’t panic, I’m a mechanic for over twenty years, and very shortly you’ll be nailing this brake job like a pro!
To depress a rear caliper, first ensure the parking brake is placed in service mode, then use a brake caliper wind back tool to twist and push the rear caliper piston simultaneously.
In this post, I’ll show you everything you need to know to get that piston moving.
Rear brake calipers are different from front calipers, in most cases, the back calipers perform two functions. They break the rear wheel but also incorporate the emergency brake (parking brake) mechanism.
The emergency brake as you may know is designed to be independent of the foot brake system.
In the unlikely event of foot brake failure, the emergency cable/electric motor-operated system is still online.
The emergency brake is also known as E brake or parking brake. The emergency brake is either standard manual cable operated or it’s electric. You can tell which you have by how you apply the brake.
If your parking brake is operated by a push-button, you have an electronic parking brake (EPB). If it’s a manual lever or foot E brake, you’ve guessed it, it’s a manual type emergency brake.
All manual emergency parking brakes are cable operated, electric parking brakes come in a couple of common flavors.
Both electric types include a dash-mounted push on/off button, controller, and either a motor at each rear caliper to apply the brake or a single motor attached to twin cables that attach to each rear caliper e brake lever.
Working on electronic parking brake (EPB) equipped vehicles will require placing the EPB controller in service mode. Some cars will allow an override by holding the EPB button for a certain amount of time. (google your model)
Most vehicles will however require a handheld scan type tool to set the EPB module to service mode. The rear pistons will not depress while the EPB is in active mode.
Working on a manual type parking brake simply requires that the brake is set to the off position.
Brake Wind Back Tool
As you now know rear caliper pistons won’t simply push back into the caliper in the same way the fronts do. The rear will need to be twisted and pushed at the same time. Difficult to do without the wind back tool, but it’s not impossible.
You’ll notice rear caliper pistons sport recesses to receive the wind back tool dogs. A MacGyver type could use pointy nose pliers to twist the piston while using channel locks to depress the piston. Awkward, but will work.
The wind back tool however isn’t expensive and makes this whole process effortless, and if you want to go the pro route, there’s an air-operated kit that most shops use. You can check out the wind back tool here on the brake tools page and if you need brake system parts, check out the Amazon link below.Amazon Car Brakes
Seized Brake Caliper
If you have tried to wind back the piston, your e brake is off and the piston still won’t move. It is possible the piston is seized. A seized caliper piston isn’t however very likely on both rear wheels, but a single seizing piston is common.
A common cause of a seized rear caliper is a badly corroded rear piston. Brake fluid is hygroscopic, it attracts moisture. Moisture in the brake system causes two serious problems, brake system corrosion and brake pedal fade.
It is recommended to change brake fluid every three to four years to prevent brake system failure.
When a caliper piston won’t move and the e brake has been eliminated as a possible issue, two remaining causes are possible. Either the piston is seized or there’s a restriction in the rubber flexi brake hose.
The following test will reveal where the problem lies.
Faulty Brake Line
Note, when performing the following test the hydraulic brake line system will be open. Opening any brake line will require purging air from the system, it’s known as bleeding the brakes. You can find a complete post about it here, “Bleed all brakes”.
To test for a brake hose issue, open the air bleed nipple and twist and push the piston as before. If the piston now moves and fluid squirts from the bleed nipple, your brake hose is faulty and needs to be replaced.
If your piston still won’t move and no fluid squirts from the bleed nipple, your caliper piston is seized and will need to be replaced. Repair kits are not available and caliper repair shouldn’t be attempted.
You may find the brake resources page useful, it’s a fluff-free guide to DIY brake repairs.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.