So you were able to work on the car when they were locked up, you disconnected the brake line from the master, and immediately the caliper released? That means the master cylinder is not working right.
Does your car also have ABS?
There are a few possibilities ast to what is wrong with the master cyl. One potential cause is that the pedal doesn't return back all the way when you take your foot off. Next time you are able to make this happen, try to lift the pedal up and see if it makes any difference. If not, there are two more possibilities: the master cylinder piston is binding intermittently, or there is no clearance between the master and the booster or the booster and the pedal.. If someone replaced some of these components and didn't notice lack of free play, the master cyl piston doesn't slide back all the way and then you may have binding issues.
Without any further info, my best guess is master cylinder has a valve problem or a piston is sticking. If you want to dig in deeper, you can take the master out and take it apart for closer inspection. Take note of how the parts came out. You may find broken seals or dirt inside. A remanufactured master cyl shouldn't be too expensive, I would consider putting one on if you have a good cause to suspect yours being bad. Sounds like you have the mechanical skills and tools required for the job.
It has to be the master, because you had fluid under pressure. The caliper can't do that. With your foot off the pedal, there cannot be pressure in the system (when it's working right) because the master would have an open passage to the non-pressurized fluid reservoir. Nothing from the caliper end pushes the fluid back or can affect the master. You could also crack a bleeder screw open at the caliper when it is binding, and you should see fluid squirt out and the caliper would release. Maybe worth a try. That would definitely tell us that it's NOT a bad caliper, and make us more certain it's the master.
There is one more thing. A little unusual on a vehicle this new, but a rubber brake line can swell shut on the inside with no signs of trouble outside. It would trap fluid in the caliper and make it bind. The master cylinder brake line fitting would NOT be under pressure, and if you crack that open, the caliper would not release. As I understand from your description, you did this, and it releaaed, so the rubber lines are good.
The proportioning valve can only reduce the pressure going to the rear brakes (or pass it through full blast). It doesn't affect the fronts.
If the rear shoes were adjusted too loose, the pedal would travel deeper, but the front brakes would grab first.
When the pedal goes to the floor, the pistons in the master cylinder travel to the bottom end of the bore, where the pistons haven't been in a long time. There can be dirt or corrosion down there that can cause the piston to get stuck and not come back.
Pump bleeding an old car is a little risky because of this. The corrosion may cause a tear on a piston seal that could sometimes cause the seal to get dislocated and cause the piston to bind. That could be an intermittent problem that comes and goes. Or a foreign particle floating in the master that sometimes gets between the piston and the cylinder bore, causing the piston to stick or leak. These are just my best guesses, based on what I have seen before. I think your master is acting up. If you want to try and fix it, you may be able to get a rebuild kit for it, not sure. If you are curious and decide to take it apart to see what's inside, you may not be able to put it back together so it would work as good or bad as it did before.. you may be left without brakes. Be prepared, don't get yourself stranded :)