Actually the RABS valve (Rear Antilock Braking System) is a common problem on that vehicle. What you can try is hitting the valve with a brass hammer (or a normal hammer, just not too hard). Sometimes that will free up the valve and get it working.
This is common as most people do not have their brake fluid flushed on a regular basis. Brake fluid (this is just for future information for you) is Hydroscopic. That means that it absorbs moisture out of the air. This contaminates the brake fluid, lowering its boiling point and the moisture causes corrosion and other problems in the braking system. On that vehicle (the Ranger) it can cause premature master cylinder failure (the seals collaspe) and the problem that you are having with the RABS valve.
Yes the valve should be allowing fluid to the rear with the power off.
The only time that it should restrict the fluid going to the rear is under hard braking and when the rear brakes are locking.
We rarely have problems with the RABS module. So rare in fact, I cannot ever remember seeing one being replaced.
Unless the master cylinder is bypassing, (the Pedal moving without heavy pressure on the pedal) I wouldn't be worried about the master cylinder. That was just information for the future. In high end vehicle (Mercedes, Audi, Jag) they require that the brake fluid is flushed at least once every two years.
I do have a brand preference, Ford Motor Company. But you have to understand, I have to warranty the part and labor. (being a dealership technician this is required, if we install an aftermarket part, we cannot warranty either the part or labor).
I would go with the new part though.