Happy you asked this before you ran into a problem. Bench-bleeding is a big help but in this case it won't work. GM master cylinders have a valve inside that will trip if the pressures aren't the same in both hydraulic circuits. This prevents losing all the brake fluid if you have an external leak, but that valve will also trip when people pedal-bleed with a helper.
With ANY master cylinder more than about a year old, you must never push the pedal more than halfway to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. When you run the pedal all the way to the floor, either during bleeding or when you're surprised by a ruptured hose, the lip seals run over that crud and can be ripped. That will result in a slowly-sinking pedal that may not show up a for a couple of days.
With GM master cylinders, running the pedal more than halfway down, with unequal pressures, as in when one bleeder screw is open, will also cause that valve to trip. When it does, no fluid will flow to one front wheel and the opposite rear wheel. A lot of people have replaced the master cylinder for that, thinking they were defective. Also, a lot of people don't even notice when that happens because thanks to modifications in suspension geometry for front-wheel-drive cars, you won't even notice when one front brake isn't working. Often the first time people ask about this is the second or third time they find the pads on one side grinding metal-on-metal, and the other side looks like new.
When I'm bleeding for other reasons, like when replacing calipers and wheel cylinders, I only use gravity-bleeding. One circuit can be bleeding while I'm working on something else, so it really doesn't take any extra time.
What I would do is fill the reservoir with fresh clean brake fluid and let it sit for a few minutes with the front up a little higher than the rear until fluid starts dribbling out of the ports. Loosen the four lines on the old master cylinder. You can unbolt the two for the rear brakes. Those are the ones that are angled down and are coming off the proportioning valves. That's the two lower lines. Unbolt the master cylinder from the booster, pull it forward, then use it as a handle to bend the two upper lines up just a little. That will prevent the fluid from running out. Install the two upper lines loosely to the new master cylinder, then bend those lines down so the master cylinder can be bolted to the booster. Install the two lower lines, and snug up three of them. Have a helper push the brake pedal no more than halfway to the floor very slowly. It should take him about 15 seconds. As he does that, you'll see air bubbles coming out of the loose fitting. Snug that fitting, then tell the helper to release the pedal quickly. Loosen a second line and do the same thing. Do each line like that, and perhaps do each one two or three more times.
By pushing the pedal down slowly, fluid will go down toward the wheels and air bubbles will float back up to a high spot. Releasing the pedal quickly will wash any air bubbles back into the reservoir along with the fluid that is rushing back up. I do this procedure with every master cylinder I replace, and I never have to bleed at the wheels
Monday, April 27th, 2015 AT 10:11 PM