Rear brakes seized

1. How many miles on the car?
2. Had they been serviced (by your mechanic or whoever) before they started getting hot?

Assuming the Prius v rear brakes are similar to regular 3rd gen Prius: there's an integrated parking brake mechanism in the rear brake caliper. The face of the caliper piston has a cross pattern, that must be oriented like an "X" when pressing against the inside pad back.

There's a stubby pin on the pad back, and it needs to be in between the spokes on the cross pattern, and firmly seated thus. Failing to accomplish this, the pin will be riding on a spoke, causing extremely uneven pressure, on one side of the brake pad, and the brakes will drag badly.

I'll attach info from the regular Prius Repair Manual, hopefully similar. Here's a screen shot, regarding that piston alignment:

 

The rears do have the added complication of the parking arrangement, as Mendel explained. If that doesn't turn out to be it, you can pick up some other info on a simple piston-return test, the availability of Toyota replacements (including -84 suffix Toyota remanufactured, if any) in this thread, though it was about a Gen 1. You can visit any of the Toyota dealers with online parts web sites to look up the part numbers, including for the rubber kit, which if I remember right was dealer priced around $30ish for all the rubbery bits to rebuild two calipers, and also included their specified red grease in a little packet.

-Chap

 

I've attached the parking brake cable adjustment instruction, but I'm thinking it wouldn't normally go out of adjustment, it's just to ensure pedal travel is reasonable.

If you have the condition of the pin not being locked between the caliper piston spokes, that could be manifesting itself in "spongy" parking brake. Every time you apply the parking brake the pin on back of pad just slides up atop the spoke. Get your mechanic to look at the condition of that pin. If it's beveled, that would indicate it's riding up.

This is what they look like new:

 

That should be unnecessary and ill-advised in nearly all cases that don't involve putting in a new parking brake cable or mechanism. Once that is installed and the cable is adjusted once, the ongoing adjustment to compensate for wear or the brake linings should be happening automatically via the mechanisms in the rear brakes.

If the parking brake ever does feel loose, the first thing to check should be whether the rear brake self-adjusters are, for some reason, not working. Only after that has been well ruled out would it make sense to turn to the cable adjustment.

And, yes, overtightening the cable would have the exact effect of making the rear brakes drag.

-Chap