Next time it locks up, open the bleeder screw to see if it releases. If it does, try that again by cracking open the steel lines at the master cylinder. If it releases there, the brake fluid is contaminated with petroleum product. We'll cross that bridge if we come to it.
If it does NOT release when the line is opened at the master cylinder, look at the left rear rubber hydraulic hose. There's a metal bracket crimped around it in the middle. Rust can form in that crimp that constricts the hose. Open it up just a little with a large pliers.