when I deleted ABS, the proportioning valve didn't allow any fluid movement at all to the rear brakes. 88 C1500.
The proportioning valve cannot close of fluid flow to the rear brakes.
there just wasn't enough pressure without the ABS unit to actually open the valve to the rear,
The ABS cannot INCREASE pressure to the rear brakes. It can reduce pressure, but not increase it.
keeping the ABS is worth it to avoid chasing the headaches you'll encounter trying to get rid of it.
...as long as there's nothing wrong with the ABS or the electronic controller. There are two solenoid valves in the RWAL anti-lock, if the one that's normally-open gets stuck closed, the rear brakes won't get fluid pressure, the pedal should be high and firm but the stopping power will suck. If the one that's normally-closed is stuck open, the brake pedal will be soft, lots of pedal travel, and crappy stopping power.
Yes, the early (iron body) RWAL ABS should have a single bleeder screw on it. The later aluminum-body RWAL does not have a bleeder screw. And, of course, the later 3-channel ABS units need a scan tool for bleeding.