I am working on a Buick station wagon. I bought it ,it had no brakes at all. All the brake lines going in to the brake proportioning valve were rusted thru. I have replaced them all and when I try to bleed the rear brakes i just get a dribble or nothing at all. I read that a button pops up on the proportioning valve when the master cylinder is empty on one side. This prevents the rest of the brake fluid from leaking out on the other side of the master cylinder.Then it says you have to take the brake line off closest to the button and go in with a small needle or screw drive and pop a little piston back up that drops when you run out of brake fluid so the remaining front brakes will still work. Any body ever heard of that? Thanks Bruce.