Thanks for finding that, Nate!
I'll just copy/paste that plus what I wrote on the first page of that thread:
"...the problem is likely the master cylinder. Of course, you wanna make sure the ABS module is bled. I've always bled as best as I could the traditional way, except ever so often I'd turn the key to "on" just as I started to push the pedal down. I'd pump a couple of times and repeat this a few times. I've never had a helper when I bled. I've always hung a plastic water bottle with a clear hose going through the cap all the way to the bottom of the bottle and the other end of the hose slipped over the bleeder. Anyway, I bleed them like this cuz the ABS solenoids are cycled during a test when you first turn the key on. If I feel it needs more or I'm just bored, I take the car to a grassy lot and hit the brakes to activate the ABS a few times, then bleed again. If you still have a soft pedal, change the master cylinder.
I just did a brake job on my '08. All the corners bled fine until I got to the last one (front left). The pedal suddenly felt odd during a stroke and would never get firm again. I started the bleeding process all over again and there was air again at the back calipers and the pedal just would not firm up. After some interwebz digging, I rushed to O'Reilly's for a new master cylinder (it was only $10 more than the remanufactured one and included a new reservoir). This fixed it. I couldn't believe that it was pure coincidence that the MC just so happened to have so many strokes left in it and they were used up during that bleeding process. This was kinda the case, but not exactly. Over time, the bore and piston seal(s) inside the MC wear just in the area where the piston has been cycling for the past 1xx,*** miles. When bleeding during a brake job, the piston is pushed much farther than it's normal travel and into the rarely- or never-touched area of the bore where it's tighter. The aged seal(s) on the piston catches in the smaller/tighter bore and gets torn or peeled out of place. Some mechanics place a block of wood behind the brake pedal to limit it's travel during the bleeding process.
A computer makes it easier and removes all doubt. But, I don't see how what I do isn't just as effective. Just as I begin to apply pedal pressure I turn the key. So, the fluid is already trying to push towards the calipers before the ABS solenoids are opened. As soon as the solenoids open, the fluid, air moisture, etc. is pushed out of the ABS module and further down the lines, followed by the fresh fluid coming in from the MC. When I flush a brake system, I do it mostly through the right rear at first. I let the bottle fill with the old brake fluid (which is darker) until the fluid coming into it begins to get lighter. By this point, there shouldn't be any bubbles. I then hang a second bottle on the left rear and bleed both rears simultaneously until they are only flowing the new fluid. Of course, while I'm pumping the pedal I'm doing the "key on trick" every few sets of pumps. This method has never failed me and I've gotten a firm and very responsive pedal every time. GMs are known for having softer pedals even when new. Replacing the flex lines with braided steel lines works wonders and running a hydrabooster will make it feel like you could stop on a dime while towing a camper."
So, Randy, when you were bleeding, did you push the brake pedal to the floor? If so, and you lost front brakes, then I'm betting you killed the master cylinder. If your further testing leads you to replacing it, be sure to thoroughly bench bleed it (it should come with fittings and hoses to do this) and leave you original one on place until you're ready to swap in the new one. You wanna have the lines disconnected for as little time as possible. When you go to bleed it, put a 2x4 under the pedal so you don't bottom it out. Also, I recommend getting a new one instead of a reman'ed unit, especially if the higher cost is as minimal as it was for my '08. This way you can at least avoid a botched reman job.
Oh- I don't know if it's possible to screw up on these brakes, but you do have the calipers on the correct sides, right? At least on other cars, I've heard of people having them swapped which put the bleeder screws on the bottom of the caliper, making it impossible to get the air out.
