RF brake locked up. Why?

You say everything is new. Does that include lines, hoses, etc.? It sounds like something sprung a leak, and brake fluid got inside the drum. Possibly a blown out wheel cylinder? Top off the master cylinder and have someone press on the brake pedal while you are at the wheel looking for a leak. If you don't find a leak, then replace what needs to be replaced and have someone step operate the pedal slowly up and down (leaving 15-20 second intervals without brake pedal pressure so you can test spin the drum).