What causes a seized brake caliper piston

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What causes a brake caliper piston to seize? More of a generic question.

Is it due to moisture in the brake fluid causing rust on the piston or cylinder wall? Or the piston boot to be damaged and allowing water and dirt into the piston & cylinder area.

In this case assume a steel piston.
 
What causes a brake caliper piston to seize? More of a generic question.

Is it due to moisture in the brake fluid causing rust on the piston or cylinder wall? Or the piston boot to be damaged and allowing water and dirt into the piston & cylinder area.

In this case assume a steel piston.
Both can cause it.
 
Another cause is seized caliper bolts. They will prevent the caliper from retracting, increasing heat. The piston will act as if it's stuck, or actually stick because of heat galling.
 
in my case , 2003 silverado , the phenalic pistons in the caliper supposedly absorb moisture and expand , which causes them to get bound in the caliper , locking up and cant move
 
Is it due to moisture in the brake fluid causing rust on the piston or cylinder wall? Or the piston boot to be damaged and allowing water and dirt into the piston & cylinder area.
I think that the piston O.D. and bore diameter are (designed) within such a tight tolerance that anything that affects it will cause it to not slide in and out "perfectly" make it a bad caliper.
 
OP, here's an example of the 2nd of your listed reasons.

Car is a 00 Volvo, well maintained 20 years in the mild (but wet) PNW coastal/lowland climate. First time off the car/out of the calipers.

Even with intact piston boots, enough grit (I believe this is mostly all asbestos/organic pad dust) gets in and sticks to the diameter that is exposed when the pads wear down. Over time that's going to lead to failure even with intact boots and well-maintained fluid.

IMG-3368.JPG


I cleaned them up with crocus cloth and while no one would have ever complained before, the braking was much more responsive and with much better feel. I was having no issues but I wanted to install ceramic pads to cut the dust, and also paint the calipers.

IMG-3369.JPG
 
OP, here's an example of the 2nd of your listed reasons.

Car is a 00 Volvo, well maintained 20 years in the mild (but wet) PNW coastal/lowland climate. First time off the car/out of the calipers.

Even with intact piston boots, enough grit (I believe this is mostly all asbestos/organic pad dust) gets in and sticks to the diameter that is exposed when the pads wear down. Over time that's going to lead to failure even with intact boots and well-maintained fluid.

View attachment 130571

I cleaned them up with crocus cloth and while no one would have ever complained before, the braking was much more responsive and with much better feel. I was having no issues but I wanted to install ceramic pads to cut the dust, and also paint the calipers.

View attachment 130572

So if you were able to clean it up and end up with the chrome plating intact with no rust or putting them must have been road crud and not rust! Did you use a new boot/seal?

How do you know the braking improvement was due to the cleaned up piston vs new brake pads?
 
What causes a brake caliper piston to seize? More of a generic question.

Is it due to moisture in the brake fluid causing rust on the piston or cylinder wall? Or the piston boot to be damaged and allowing water and dirt into the piston & cylinder area.

In this case assume a steel piston.

The piston boots don't need to be damaged to let water in, but it helps!

Usually seized pistons are rusty inside the caliper aswell, and it's not just the piston rusting, in fact it starts with the caliper rusting. Piston rust isn't an issue as long as the chrome finish is intact.
 
Running excessively thin brake pads for long periods causes the piston to remain in an extended position further outside the caliper bore/boot. Dirt, rust, and the fact that the piston is more prone to “cock” off center will create a seized piston. In my opinion…
 
The piston boots don't need to be damaged to let water in, but it helps!

Usually seized pistons are rusty inside the caliper aswell, and it's not just the piston rusting, in fact it starts with the caliper rusting. Piston rust isn't an issue as long as the chrome finish is intact.
And what causes this rust inside the caliper? Moisture in the brake fluid?
 
The piston boots don't need to be damaged to let water in, but it helps!

Usually seized pistons are rusty inside the caliper aswell, and it's not just the piston rusting, in fact it starts with the caliper rusting. Piston rust isn't an issue as long as the chrome finish is intact.
Sometimes a brake hose will collapse on the inside and won't let the brake fluid go back in when you let off the brake pedal.
 
So if you were able to clean it up and end up with the chrome plating intact with no rust or putting them must have been road crud and not rust! Did you use a new boot/seal?
No, polishing takes care of the former (crocus cloth is abrasive, and I used polish). Re-used the same seals and boot, no issues w/them.

How do you know the braking improvement was due to the cleaned up piston vs new brake pads?

Because the replacement pads were less capable than the higher-performance Jurid ones I took out.
 
Pistons seizing or partially seizing in the caliper is particularly a problem on motorcycles because of the increased exposure to rain and salt. I've solved the problem in both bikes and cars pretty effectively using one of the modern silicone based brake greases. I lube the pistons during assembly and will occasionally remove the rubber boot and apply further grease. Even before total seizure occurs, the benefit of keeping the piston lubed is it allows the piston seals to retract the pistons when the brake is released, which reduces drag on the discs.

Brake Seal.jpg
 
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