Brakes lock up

After driving about 30 minutes or so, the brake pedal gets very high and hard, and the brakes are "on" all the time. Made evident by having to drive with accelerator hard down; "brake pad" smell; noise from wheels; jerks to final halt at lights even without pressing brakes; getting about 5 mpg. No pulling to one side. Brakes certainly work (too well!)
I have replaced pads, rotors, calipers, wheel cylinders, master cylinder, all to no effect. Seems to be more prevalent in hotter weather. After leaving it stopped for an hour or two, all back to "normal" till driving for another ~30 minutes, then symptoms recur.
The car has rear ABS. Could this be causing it, or something else? (Very expensive to "fix" ABS)