Makes me wonder a bit. Post a photo. An adjustable proportioning valve has a single inlet and outlet and is plumbed only in the rear tubing. If your front tubes go to it, it is a "combination valve" and much more complicated. The proportioning valve decreases pressure to the rears, so no way you could adjust it other than your statement. Regardless, both rear wheel cylinders should see the same pressure, so the left locking up and not the right is likely one of the stated hypothesis (gunk on shoes, wrong shoe assembly, adjustment, ...).