2010 Gen 3 rear brakes both locked up

Although I am NOT familiar with the Gen 3 models, the parking brake is actually (usually) a small drum brake located in the center of the disc. (not certain if this is the arrangement on a 2010 Prius) These drum brakes are operated by a cable attached to either a foot pedal or a hand operated lever in the center console. Most likely the cable has stuck.

 

There is no drum brake on third gen Prius. There is a cable running from the parking brake pedal to the rear brake, that rotates a lever on the back of the caliper cylinder, operates a sort of cam to lock the rear brake. It could be that cable is overly tight. Maybe.

The US schedule calls for a visual inspection every 5K miles or 6 months, and also a full inspection, including pulling off the caliper, every 30K miles or 3 years. Have you been doing the latter?

There's one gotcha with the rear brakes, that can cause them to drag: due to the incorporated parking brake, the rear brake caliper piston can be moved back into the bore only by screwing it in. There is a cross pattern on the face of the piston to help with this.

There's also a need to prevent the piston from rotating, whenever the parking brake is used. Toyota accomplishes this by having a stubby pin on the pad back that is supposed to be positioned, and locked, between the raised spokes on the piston.

It is very easy for this to not work as planned. DIY brake inspection might overlook this piston orientation requirement, or the piston might be positioned properly at reassembly, but not well seated thus.

However it happens, if the piston manages to rotate, so that a spoke on the piston face rides up on the pin, then you will have skewed braking pressure, and constant drag.

This will be evidenced by scored rotors, grossly bevelled wear, in particular on the pad next to the piston, rust on about 50% of the inside face of the rotor (due to lack of pad contact), and constant brake drag.

To avoid this: install the caliper with the piston oriented thus:



Then press the brake pedal multiple times to seat the caliper piston against the pad, then reconnect the 12 volt negative cable (you should disconnect this before doing any brake work, to play it safe), take the car for a short drive, and back in your driveway/garage apply/release the parking brake several times (for the first time after the brake job), chock the front wheels and raise the rear of the car, then spin the rear wheels. They should go 2~3 revolutions with a good push, not free-spinning, but reasonably easy turning, with just light drag.

 

As I understand it, the v (which is kinda, sorta, a third gen) has the top-hat style rotor with little bitty brake shoes for parking. But the Gen 3 liftback, as Mendel describes, has rear discs only, and the parking brake cable acts on the calipers.

The OP said the BRAKE warning light was illuminated, and the switch making that light up is right up front attached to the parking brake foot pedal mechanism, so whatever was binding up, it must have been preventing the pedal from coming back up to the fully-released position.

 

Yeah good catch.