No fluid to rear brakes

At some point you pressed the brake pedal too far. That must never be done on any car over about a year old. Crud and corrosion build up in the lower halves of the two bores where the pistons do not normally travel in the master cylinder. When the pedal is pressed more than halfway to the floor, either during bleeding, running the pistons out of the calipers, or when surprised by a sudden leak, the pistons ride over that corrosion and the lip seals can get ripped.

You have an additional problem that only involves GM front-wheel-drive cars. If you look closer, you will see you are indeed getting brake fluid from one rear wheel and only the opposite front one. When the brake pedal is pushed over halfway, a valve in the master cylinder trips to block one front port and the opposite rear one. A lot of people replace the master cylinder for this, then have the same problem.

The only way I have found to solve this is to close three of the bleeder screws, loosen the cap on the reservoir, open the front bleeder screw on the caliper that is not flowing fluid, then give a quick, short burst of compressed air into that bleeder. Let the fluid gravity-bleed, and gravity-bleed the other three wheels.

If you still need to work the pistons out of the calipers, do that by pumping the brake pedal, but never over halfway to the floor.

Sorry for the delay in getting back here. Do not jump on the master cylinder yet. As for which wheel to give a spurt of air, I always have done it to the front one that is not flowing any fluid. The front calipers are always a lot larger than the rear calipers or wheel cylinders, so there is less chance the compressed air will make it all the way up to the master cylinder. All you are after is enough air to unseat that valve that tripped in the master cylinder. I hold a rubber-tipped air nozzle to the bleeder screw, then give a quick, sharp pop to the lever with the side of my hand. The air only goes perhaps 4 to 6" up the line, and that gravity-bleeds out in less than fifteen to twenty seconds.

Funny thing is I just popped a rear brake line today on my minivan. Took me twenty minutes to replace it, then I was able to bleed it right at the joint, and since I had to pump half of the reservoir empty to get home, there was air in the front too. I got that out by filling the reservoir, then pushing the pedal down only halfway very slowly so the air could keep floating back up, then I released the pedal very quickly. The brake fluid rushing back washes the air up and into the reservoir that way. Tonight the pedal is solid again and I can stop whenever I want to!

If that box you are referring to is bolted right to the master cylinder, that is the hydraulic controller for anti-lock brakes. I never ran into this problem on a GM front-wheel-drive car with anti-lock brakes, but the same solution applies. I would try the compressed air procedure at one of the wheels where no brake fluid is flowing. If that does not work, you may need a scanner to send the valves "home" in the hydraulic controller. This ABS system is very inexpensive and quite effective. The three valves should home automatically, but nothing will happen unless the ignition switch is turned on.