1993 Mustang, 5.0, standard shift, bone stock unit.
Rear brakes are not working. New pads, drums, fluid, bled properly and adjusted properly trust me. The brake booster is not leaking and the check valve and manifold vacuum is good.
No brake pedal fade at all. Brakes do not stop the drums even with the wheels off, just the drums on and secured with lugs.
Backed off the brakes a bit to remove the drums, fired up the engine, just touch the brakes and the wheel cylinders do move outward.
Now the general opinion is that the proportioning valve is the culprit. BUT the proportioning valve is designed to allow MORE fluid to the rear brakes say if you break a front brake line, or if you do a panic stop. The proportioning valve allows extra fluid to flow back to the rear brakes.
My gut tells me that the master cylinder rear reservoir for the rear brakes is bypassing. It sends just enough pressure back to the rear brakes to move the unimpeded rear wheel cylinders BUT won’t move the wheel cylinder pistons once the brake shoes/drums are on.
Easiest way to test my theory?