Part of the reason [in my case] that I suspected boiling fluid was that the temperature that day was over 100. That's pretty hot for around here. It was about 4:30 PM, again, the hottest part of the day. The truck had been parked in the sun, I started it up, drove a couple of miles and crossed a bridge. When I came to a stop at the bottom of the bridge, I had to give it a little more throttle than normal to get up to speed and the transmission was not shifting into gear like it should. I let off the throttle a couple of time and felt the truck dragging. Drove a few more blocks through town playing with the brakes, trying to get them to release. After a bit, came to the conclusion there was no way I was going out on the highway with the brakes dragging like this. Pulled into a parking lot, turned the wheel and confirmed my front brakes were both hotter than hell. With no other reason, other than the heat, I surmised that I still had pressure on the calipers, but did not know why. Got a wrench out of that wonderfully little stash spot in the back and cracked the bleeder. I got a squirt of fluid out of both side like someone had their foot on the pedal. Just one squirt and they were done. Got in the truck and have had no other problems with the brakes since. If it was a mechanical problem in the calipers, it would not have been both. Problem in the master cylinder? I would have thought the back brakes would also have drug. Issue with the ABS part of the system? Maybe, I don't know it well enough, but I would bet money it had something to do with heat that day and not flushing my fluid. It may not have boiled, but something happened that applied pressure to the front calipers and would not let them return.
Hard lesson learned, flush your fluid. It's cheap and easy insurance.
Sorry if this is a little long winded, just wanted to explain my "methodology" if you will.