Hi guys. KHLOW2008 was referring to the damage that can occur to the lip seals in the master cylinder when the brake pedal is pushed over half way to the floor. That should be avoided with any master cylinder that is more than about a year old, but on GM front-wheel-drive cars, that is what trips the valve and blocks two ports, presumably the one with an external leak and its diagonal opposite.
A lot of people believe that valve won't trip if you pedal-bleed in a specific sequence, but if you think about it logically, that valve trips for a leak in any of the four circuits, so it will trip when any of the four wheels is bled first.
The only way I have ever found to reset that valve is to go to one of the two wheels that isn't flowing any fluid, open just that bleeder screw, then give it a short, quick burst of compressed air. I use a rubber-tipped air nozzle, and give it a quick whack with the side of my hand. The cover should be loose on the reservoir. After that, just let it gravity-bleed. Once you have fluid flowing from all the wheels, and the bleeder screws are closed, "irritate" the brake pedal a little, by hand, then open each bleeder screw one at a time again to let the last few remaining air bubbles out.
Tuesday, March 7th, 2017 AT 9:00 AM