What exactly happens when a caliper is seized?

[sugarwood]

My brakes work perfectly, but I've always meant to clarify this question.
Pardon the stupid question.

What exactly happens when a caliper is seized?
Do you have no brakes
or do you have brakes stuck on (wheel gets hot) ??

First, take the scenarios of "brakes are stuck on". How exactly would that work?
Brake pedal pushes the piston, pad pushes against the rotor, and then the caliper gets stuck? But, if the pedal is released, how can the pad keep applying pressure to the rotor? The pad would wear off that top layer and then be clear of the rotor.

So, I have no idea how a seized caliper can cause brake wear. Even if the caliper could only move in one direction (piston can move out, but not back in), As soon as pedal is lifted, pressure is off, piston is not still pushing the pad. If its stuck pressing the rotor, won’t the pad wear off, and then you just have no brakes?

If anything, shouldn't it cause a no brakes situation? Caliper piston is stuck inside the caliper. Brakes pedal can't move it, so no brakes. Not brakes stuck on.

Which is it?

[fred cook]

With a seized brake caliper, usually what happens is that the piston gets stuck in the bore because of corrosion. Remember, brake fluid is anhydrous (absorbs moisture). So, when the brake is applied, the piston presses against the brake pad and thus the rotor. However, because of the corrosion, when the brake pedal is released the piston does not retract keeping pressure on the pad. After being driven for a while, the pad will wear down releasing the rotor, but the next time the brake pedal is pressed the wear and excessive friction starts all over again. In severe cases, the heat generated can be enough to cause the brake fluid/water moisture mix to boil causing the brake pedal to go to the floor when pressed! Not much fun when you really need to slow down! Another thing that can cause a similar problem is that the flexible brake hose can deteriorate and cause the caliper to hold pressure. Either way, it's time for some brake work!

[Coastr]

Most likely the caliber is stuck open from sitting. Brakes still work as you have 3 others (maybe). Hard braking will lock up the ‘good’ wheel because the other is not doing its job.

Stuck calipers are usually neglect and a sign to do everything in the brake system - do it once and do it properly. A Porsche should stop very well.

[sugarwood]

My brakes work perfectly, but I've always meant to clarify this question. OK, so Fred is outlining a scenario where the piston can be pressed out, but can not be retracted. This makes sense since there is a lot of pressure pushing the piston out, but not really sure what pushes the piston away from the rotor besides friction. So, if the piston is sticky, maybe it does not retract as well. In that case, yes, each time you press the pedal, the piston will stick and wear down the brakes more than usual.

[Bill Verburg]

You will most often have brakes and not notice it for a long time, what usually happens is that only 1 of the 2 pistons in a given caliper rusts in place, the other side piston will usually continue to work . One side pad wears and the other doesn't

[Fairview]

Hey Sugarwood. It may not be the caliper at all.

My experience with stuck brakes (brakes come on as you stop but won't release or only release partially) is when the rubber flex lines from the body to the wheel/suspension are ancient and are internally compromised. So the brake fluid can push through the rubber flex line under the high pressure of the master cylinder, but then the fluid cannot flow back from where it came, keeping the pads in contact with the rotor.

The inaugural experience was with a 1961 Rolls Royce Silver Cloud II. I learned to take a 7/16" wrench with me when the right front brakes would stick. I could pull over when I felt the brake dragging, get out, lie down on the pavement, reach up behind the drum and open the bleeder screw with the wrench. A spurt of fluid would come out, releasing the brakes, and I'd be on my way..... finally I figured out it was the rubber flex line to that wheel that was causing the problem. I'm glad I got over owning British cars!

[1979-930]

Originally Posted by Bill Verburg

You will most often have brakes and not notice it for a long time, what usually happens is that only 1 of the 2 pistons in a given caliper rusts in place, the other side piston will usually continue to work . One side pad wears and the other doesn't

This and usually people never know until one pad goes metal to metal. With 4 piston calipers you can get a pad that is worn at a heavy angle due to only one piston sticking.

It’s really rare that the caliper sticks enough to get an over heat condition that causes brake loss. If you did you would feel a pull to one side if it was a front caliper. If it’s in the rear you would probably not feel it.


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[jlex]

One of the most telling ways to determine if you have a dragging caliper is to note how it rolls at slow speed on a level surface. If the car stops short of a normal coast, start looking for a bad caliper by jacking the car up and spinning each wheel.
It's the internal seal which causes the piston to retract into the bore. If the seal is compromised, it doesn't take too much corrosion to disallow the seal to do its work.

[djpateman]

Generally when a piston seizes in a caliper there can be two situations. First is total seizure and the piston never moves and the pad will not act on the pad, nor the pad on the rotor so braking action is reduced. Second is partial seizure where hydraulic pressure can force the piston out, but the piston is not able to retract. In this case the pad is forced against the rotor and must be worn down until the rotor is no longer dragging on the pad. This will result in hot wheels and lots of brake dust. I had one interesting caliper (came to me to repair) where the piston partially seized and the piston face was poking through the metal backing of the pad and wearing on the rotor; it could go no further since the piston was past the seal and the caliper leaked rather than pushing the piston out further.
Often there are 3 things at work to retract the piston. The rubber seal ring distorts as the piston is pushed out, and this acts to pull the piston back. The brake rotor will wobble a small amount and push back at the pad to create the venting clearance, and in so doing also acts to push the piston back. Early calipers in the 60's and 70's had a pin in the back of the bore, and an internal mechanism inside the piston that was spring loaded; the mechanism gripped the pin, and as the piston was pushed out the spring was compressed, when pressure was released the spring drew the piston back.

[Mixed76]

Originally Posted by fred cook

Remember, brake fluid is anhydrous (absorbs moisture).

Just for clarity, anhydrous means without water and describes the current state of something, not a characteristic of the material.

Hydrophilic means water loving (mixes with, dissolves in, or absorbs water), hydrophobic means water fearing (does not mix with, dissolve in, or absorb water). Both of these are characteristic of materials and generally don't change.

Brake fluid is made anhydrous (water is removed) by robust chemical processing, but it is naturally hydrophilic.

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[Mike Billings]

I drove a few mile miles with a stuck caliper on a GM car. It was braking very slightly all the time.

After stopping, a small fire popped up on the caliper with 6" flames.

The gas station I stopped at couldn't find a fire extinguisher so I put it out with one of those tiny 4 oz courtesy water cups from the drink station.

[djpateman]

The ultimate seized caliper situation I saw what posted here on Pelican where, on a domestic vehicle, the rotor was worn down to the internal venting vanes on one side. At which point the piston probably popped out.