I have never seen a proportioning valve, or combination valve, that has an independent return for fluid back to the MC. They simply direct fluid, and thus pressure, proportionally and flow both ways. Likewise residual valves - just a pressure regulator with no alternate flow path. The MC creates the pressure in the lines to the front & rear brake lines, and the only path back to the reservoir is those same lines back through the return ports. If the pedal goes to the floor, it can only be a bad MC, unless I'm missing something. My guess is that you've been running on front brakes only for some time (seals on the rear spool bad), and now the front one is starting to fail, but sealing part of the time.