My guess would be that one of the rubber lines collapsed upstream. You could try cracking open the connections at each rubber line working backwards until you confirm a blockage or until you prove there is none. My next wild *** guess would be something's wrong with the ABS, but I hear so many bad things about GM rubber brake lines that I would start there. BTW, I could NOT gravity bleed the rear brakes on my 96 S10, but those were drums. I had to do the pump the pedal method to get them to bleed.

 

Update: Was unable to get the bleeder off on the front right caliper so I had to replace that in order to bleed the entire system. I did 4 rounds of bleeding by myself before I had clear fluid and no air bubbles all around. The amount of air that made its way out of the Left rear was insane. Once that was done, pressing on the brake gave my piston pressure all around. Test drove it and did some 30mph slams on the brakes to set everything and they feel great! Thanks alot guys.

If you are doing this for the first time and are using the clear tube/pop bottle bleeder trick, make sure the tube and the bottle are above the bleeder. This absolutely ensures no air makes it back in when you de-press the pedal.

 

I prefer 2 people method

 

As do I...when a second person is available. I only work on my cars at night and waking up the wife to assist in a brake job is just asking for it.

 

Thats why I had kids LOL

 

When I have done the rear brakes on my two Blazers, I only had success when I gravity bleed the rears; each side for a good 15 minutes and was good to go. On the fronts, gravity bleed did not work for me, and had to use the two people method. Also tried using the hand vacuum pump....just too much air leak around the bleeder screw threads.

Gravity bleed is the best.

 

You have proven once again that bleeding the rears takes lots of fluid and time because so much line is involved and if left open it all drains out and is replaced by air. Which then has to be bled back out. Gravity bleeding works when you have a caliper off too.
If you stuff a golf tee into the open end of the line it stops the leaking and reduces the amount of bleeding required afterwards.

You have also proven that unless you mess with the ABS you won't have to have it bled by someone with a Tech2. I have a Tech2 and have never needed to bleed the ABS with it because I'm careful not to tamper with the ABS while doing brake work.
The old don't - fix it till it's broke.