Brakes distribution block/proportioning valve question

dunno about "mushy", but the 2 things a proportioning valve does is 1) limits the max pressure to the rear brakes so they don't lock before the fronts and make the car unstable, and 2) has a delay in the pressure application to the front brakes, so the rear can take up their clearance and start braking first, which helps reduce nosedive. the mushy is probably due to air and need to rebleed. if you drained the MC fully, you may have to bleed that separately first to get all that air out.