I have had a similar issue a couple times with other cars when the caliper puck did not retract sufficiently.  One time it was cured by exercising the caliper (collapse with C-clamp and pump up again a few times).  The other time I installed rebuilt calipers when the exercising did not work.  Warped rotors can act like this too, but thankfully I never experienced that. 

I suspect what you are referring to as a  proportioning valve is that little rectangular brass device with a wire hooked to it, it is a switch for the brake trouble light and should not cause this.