Do Plastic Calipers Jam Up?

Can you post pictures of the caliper in question?

 

Usually when I do these all I do is take my channel locks and rotate the piston in the boar assembled with the bore and the piston and the seal and the rubber and all on I just grabbed the piston and turn it say a half a turn of the diameter of the piston and then squeeze it down with the chanel lock pliers. That usually frees up any funny business unless everything work on reassembly I'm not sure what's happened here obviously you didn't think you need to bleed the brakes maybe that's something that needs to happen on that one side possibly but generally no the composite Pistons don't generally seize in the boars because they're not rusting and rust even if the caliper metal was rusting it can't stick to the composite piston

 

Not quite all Prii. The pistons were metal in gen 1, and the phenolic pistons were introduced to Prii in gen 2. (They go way further back in some other vehicles,)

The pros of phenolic pistons are: they don't rust/corrode, and they're much less heat-conducting than metal pistons so less braking heat is carried back to the brake fluid. One con is that they can (at least the early formulations) swell, and become tight in the bore that way. The swelling seems to be pretty rare and happen only in very special circumstances; ordinarily they're very trouble-free. I do remember at least one PriusChat thread though where there was a swelling problem.

This study appears to suggest that it's dampness/humidity that swells them (when they swell). The problem was pronounced in RVs, which can sit unused for long periods soaking up damp, and don't see enough regular use to heat the pistons and drive the moisture back out.

These days, if I rebuild a caliper, or even take one fresh out of a box, I will test the piston return on the bench before even putting it on a car, as described in this post. Insufficient return of the piston can cause dragging and overheating, whether it's because of rust on a metal piston, swelling of a phenolic one, hardening of the square-cut rubber seal, or any other cause.* (Then, of course, once it starts, it's going to cook that rubber seal anyway, even if the original cause was something else.) So a quick check of the piston return lets you catch that problem right away and deal with it, whatever turns out to be causing it.

* and yes, I have seen even a fresh-out-of-the-box reman caliper with no piston return. The piston and seal were both new and nothing was corroded, but the rebuilders don't always seem to get the right magic amount of friction between the rubber and the piston. In the problem one I saw, they used a metal piston with a very smooth mirror finish. In a caliper from Toyota, by contrast, even the metal piston had more of a matte surface, and the seal gripped and retracted it avidly.

 

Why bleeding ? I just undo line put rubber cap on line. Remove old caliper . Take new syringe on new fluid till it pops out of loose bleed screw. Hand close bleeder. Uncap hose it's dripping fluid offer it up to new caliper that's full . Tighten . Crack bleed see air bubbles just a few popp pop pop and now clear. Should be no bleeding now . You're I guess just letting it all run out or such?

 

your post made me laugh jim. i can remember 40 years ago, my 9 month pregnant wife pumping the brakes on our ancient valiant in the middle of winter while i bled them.

 

Definitely the seals and probably the phenolic piston. Rest probably ok.

If the injured caliper was an original Toyota, I would probably buy Toyota's rebuild rubber kit (everything you need but not the piston) and piston and put it back together, and probably have a better caliper than an aftermarket. If it was already an aftermarket caliper, then yeah, might as well replace it with another one, and let the aftermarket guys rebuild the core you send back.

 

the key is capping that brake line, when you take it off. then prefilling your new caliber.
I do a variation of that procedure with a vacuum pump, to pull fluid through, rather than rely on gravity.

Brake Bleeder and Vacuum Pump Kit

 

Well whatever you do doing one caliper at a time you shouldn't have to do any bleeding The stuff is running out of the hose when you undo it All you got to do is put a little rubber cap on it have it in my pocket when I start the job and then when I screw the new caliper on I just let it sit there and crack the fitting and wait a minute or less and it fills the caliper and comes out of the bleed screw then I just close it leave the caliper hanging and put my brake pads in in the springs and all the nonsense and then drop the caliper on go do the other side I try not to be sucking on the stuff with a mighty vac that can pull air around the threads of the bleed screw but that's neither here nor there was just another less amount of tools I have to pull out to go to the side of this car but anyway so you've got the idea and then there shouldn't be any bleeding just saves the step that isn't needed