Gravity bleeding is one of those things that works great for some and not at all for others. On my car we replaced both calipers and both wheel cylinders, pads and shoes, drums and rotors, and front brake hoses, opened all the bleeders, topped off the master cylinder a couple of times while it gravity bled for a couple of hours, and I had a great pedal. You only need to keep the plunger from coming out on the proportioning valve if you are pumping the pedal to bleed them.