I'd like some help with understanding something that happened the other day. I was leaving work, the weather was cold. Stopping at the end of the drive before getting on the main road, I had almost no brakes. I pushed harder, it felt almost like something gave, and then the rears locked up for a second. Then everything was fine. It felt like both rears locked up; the car didn't pull to one side.

I can accept that I probably have a bad caliper in the rear that's not engaging, but what made the whole system not work?
The weather since then has gotten warmer, and no problems.

Related but maybe not relevant info: This winter, with the cold, I've noticed that the parking brake doesn't like to release sometimes, and I'm dragging the rears a bit. Also, I get a grinding noise from at least one of the rears at first, more than would be accounted for by getting rid of any rust on the rotors from sitting outside. So I'm thinking at least one of the rear calipers is needing replaced. Also, I need to have the brakes bled I guess, as I don't have equipment to run the ABS. I bled the brakes last fall as well as I could, but it's still not very solid. Lastly, if I've been driving on the highway for a while, and hit the brakes to decal for an exit, it pulls to the side a little.