I posted earlier but put in the wrong spot. I have 87 suburban with 454. The rear brakes are not working. I went to bleed them and am not getting any fluid from the rear brakes. The pedal is spongy and goes almost to the floor. I am getting fluid to the rear proportioning valve that is attached to the . But the wheel cylinders are dry. I s there another valve somewhere. Is there a way to test the from proportioning valve?