Just adding information:

I would not think fire in the back would cause anymore damage up the line. But one never knows, you can check the booster. Put your foot on the pedal and start the motor it should go down some when the motor fires. Sorry to use that word but it was funny, then take your foot off and reapply the brake, it should get pressure at the top of the travel.

Your description sounds like the booster, not the proportioning valve. There could be a proportioning valve problem if the line blew out and the rear lost all pressure. It might have forced the valve to go off center and all the braking is now being done up front. There is an internal valve that will or can get off center, it can be reset go to master power web site and look for the procedure.