If you want to test that theory cap the lines going into the porportioning valve and see if your problem persists. I would encourage you to verify you have a properly working master cylinder first.
When I cap the front port of the MC (line that takes fluid goes to rear brakes) fluid comes out of the MC with ease.
While the front port is capped and fluid is allowed to flow out of the bleeder screws at each caliper (passenger and driver) fluid flows out of the bleeders with ease.
When I cap the Rear Port (line that directs fluid to front brakes) fluid comes out of the MC with ease.
While rear port is capped and fluid is allowed to flow out of the distribution block to the rear brakes, fluid flows out of the distribution block with ease.
I haven’t tried it at each wheel yet but… I think this means my MC is working properly. Although it definitely needs bled again after shooting fluid straight out of it and bringing fluid right back in.