I have had this problem. First, that's not a "reset button", it's a disable button that must be depressed to properly bleed the brakes. Second, you are correct. The problem is that the valve spool inside the proportioning valve has been pushed all the way to one side and is blocking the rear brake passages. I've been able to move it back by vigorously pumping the brakes. Failing that, loosen a FRONT bleeder screw and press on the pedal. There should be enough pressure in the rear lines to push the spool the other way (frequently TOO far, in my experience
). You might need to go back and forth a couple of times. Be gentle on the pedal.