Troubles with brake caliber

This shouldn’t be the thing that you have to ques.

Was the caliber locked or the pads just lock up in carrier? If the pads were locked it can’t be electrical problem. Probably pads were just installed badly.

If the caliber is locked it’s easy to test is it the caliber or problem in some part before it (like electric fault) by opening the bleeder.

 

please see member 'mendel leisk', many fine posts on gen III rear brakes. if you cannot locate through search function, private message him and he will be happy to help.

 

Yeah you can see my latest tirade on the rear brakes, starting at post #10 here:

May have been fleeced by the dealer - what else to look out for? | PriusChat

One thing extra: if you're not using the parking brake, and haven't for some time, there's a possibility the cable has seized up, not releasing properly. For that reason alone it's a good idea to use it at least periodically. It's also good, even on flat ground, to reduce load on the transmission's parking pawl, a pin that locks the front wheels. And it's also good because... ok, I'll stop.

The rear brakes include a parking brake mechanism. A cable runs from the parking brake to a lever on the back brake calipers. When the parking brake pedal is pushed in, the cable pulls the lever at the caliper. The lever's axle runs through the caliper housing into a screw inside the piston in the caliper.

This axle rotates, and what is supposed to happen: the screw inside the caliper piston will screw out slightly, bringing the piston along with it, clamping the caliper securely on the rotor. This is the parking brake action.

But, this screw is inside the piston, and the piston is free to slide inside the cylinder, so when the lever arm axle rotates, it's also possible that the whole piston will rotate. This is not good, means you'll get no locking, etcetera.

To prevent this piston rotation Toyota's engineers put a pin on the back of the inner brake pad, the one that contacts the caliper piston. And on the piston face in contact with the pad, there is a four-spoke pattern. The intention is for the pin to be positioned between the spokes, preventing the piston from turning.

The issue as I see it with this design comes during assembly of the rear brake components: you need to make sure that the pin is properly positioned between the spokes, and you need to ensure during re-assembly that the system is well pressurized, by pumping the brake pedal multiple times, getting the pin firmly seated and locked.

Failure to do both of the above, can end up with the pin riding up on the spoke. Now the inner pad is hopelessly skewed, making poor contact dragging badly, and the effect ricochets to the opposite side pad, the alignment of the calipers, maybe messing up the caliper pins as well.

An very easy way to monitor the rear brakes is to feel a rear wheel after an extended drive. If it's markedly warm, warmer than the fronts, they're likely dragging. A bit more involved: securely chock the front wheels, then raise the rear enough to get the tires clear of the ground, and try spinning the wheels. Apply and release the parking brake, and try again.

A slight drag is normal, the rear brakes are always on the edge of locking, due to built-in parking brake mechanism, which automatically adjusts to be just on the edge of grabbing. But if it's hard to turn, it's not good.

God, I'm hooked on this subject. I guess cause we had the issue, and I found the best way to resolve it was to understand it, lol. 8 months back ours was messed up, see the pics and story in the attachment. Now, with new rear pads, minor cleanup (with steel wool only, no machining) of the rear rotors, and careful reassembly (as outlined above), the rear brakes are fine and the rotors have gone from scored/rusty to like new.

I'll attach a segment from the Repair Manual on brakes, has the basics, torque values.

 

That's very close, as I understand it, but as long as we're hooked on the subject, it's possible a detail or two could be off.

As I understand it, anyway, the parking brake action does not rotate the threaded screw. The cable pulls a lever on an axis perpendicular to that screw, which just carries a cam, which pushes the screw toward the piston ... straight toward the piston, with no twisting movement. In fact, that screw is only able to move inward and outward, never rotate.

Behind the piston is a threaded collar that rides on that screw. When the screw gets pushed, that threaded collar pushes the piston, applying the brake.

Where the threading comes in is for the automatic adjustment of clearance, and that happens when the regular, hydraulic brakes are applied. As the pads wear, the hydraulic fluid will push the piston further out. The parking screw is fully retracted, of course. It can't turn, but the threaded collar behind the piston can, and does, so it's a tiny bit further 'unscrewed' (toward the end of the threaded screw). When the hydraulic brakes are released, the piston doesn't retract quite as far back as where it started, so the threaded collar is riding on the screw at a new position. A spring, wound around that collar in one direction and anchored to the piston, prevents the collar from turning the other way, so it can't just spin back down the threaded screw when the parking brake is applied.

The only way to spin it back down the screw is by turning the whole piston, as is done when replacing the pads.

-Chap

 

That makes a lot of sense. I hadn't got my head around that, thanks. Lever rocks a cam, pinching action of pads on rotor. And the reverse when it releases. And the screw very gradually accommodates pad wear. And the tricky spring (I knew about that) behaves like a Chinese finger trap: only allows the screw to lengthen.