Front passenger side disc brake freezes up?

: my son's 1991 jeep Cherokee's front passenger side disc brake freezes up. The brake pedal becomes stiff and can not be depressed. If I let the car set for 45 minutes, the brake returns to normal. (My son and his friend worked on it before me. The car also needed wheel bearings, which were replaced. I have replaced the caliper, and the brake hose. (The boys also replaced the master cylinder. ) The other brakes are fine. The car has 246,000 and is just broke in. Please help. Thanks, distressed dad