Help! Rear Brakes Locked Up

I got a '86 420SEL. The rear brakes locked up 3 times. All after a while driving, and when applied brake then resume driving, the gas pedal felt heavy and it could only move at most 20 mph. then braked to full stop, found the rotors coal-red hot, and the car wouldn't move at all. But after about 30-60 minute, everything normal again. Brought to mechanic, he said he founf a metal stuck in parking cable, and i drove home. Two days later, it stuck again. Went to another experienced mechanic, he drove two days, found nothing. I drove back, and the next driving only took 10 minuted to happen again. I saw someone here with same problem, but anwered with a opinions, parking cable and master cylinder, and sort of believing it's master cylinder, but should I buy one or rebuild one? Any idea welcome as well as procedure.