The way to approach this is to open the hydraulic system at various places to see where the trapped fluid releases and where it doesn't. When the brake locks up, crawl underneath and open the bleeder screw on that caliper. If it doesn't release there, the caliper is sticking. If it does release, the next time it happens, loosen the steel line at the master cylinder. If the brake releases from there, the fluid is not being allowed back into the reservoir. Two common causes of that are something is holding the brakes partially applied or brake fluid contaminated with petroleum product. If the the push rod is adjusted too long or the brake light switch is misadjusted, they will hold the brake pedal down too far. Petroleum product in the brake fluid will cause all rubber parts to swell. The lip seals will grow past the return ports and block them. That includes engine oil, transmission fluid, power steering fluid, or even slight grease residue from your fingers. The clue is the fluid pressure will release right at the master cylinder and the rubber bladder seal under the reservoir cap will be blown up and mushy.
If the fluid does not release from loosening the line at the master cylinder, look for a rear rubber flex hose that has a metal bracket crimped around it. Rust can build up inside that crimp and constrict the hose. That will let the brake apply but not release. If you find a crimped bracket like that, just open the crimped part up a little with a large pliers.