I'm at a total loss now. I have no brakes at all at the rear. Here's what I've done since I last posted.
I found that when I had removed the RWAL, I had actually cross-threaded the brake line fitting into the combination valve. So, I went to the junk yard and got a new combination valve, RWAL system, and the brakes lines between them all. I connected it all (with the RWAL back in).
I also replaced the master cylinder with another new one. This one, I bench bled with hoses until I saw no bubbles, I left it for a couple hours and did it again to make sure that there wasn't anything leaking. It looked good, no bubbles. I installed it and bled all of the brakes in the right order.
Now, I have no brakes. Before I bled the brakes, I lifted all 4 wheels and removed them. After bleeding the brakes, the pedal feels nice and firm when the truck isn't on. But as soon as I start it, the pedal goes all the way to the floor. With all 4 wheels on stands, I started the truck and put it in gear. The real wheels spin even with the pedal to the floor.
Also, the brake light stays on, if that helps diagnose.
Please help!