!988 k1500 4x4 5.7l brakes

Are you getting flow to the rear cylinders? Nevermind...you said you bled everything. Since the ABS light is on, I question if the ABS Control Module maybe the problem. I'm currently rebuilding the brake system on a 90 model Silverado. A buddy of mine replaced the master cylinder in his girls truck but errantly added power steering fluid to it which didn't work out to well. I've replaced the MC and all 3 rubber hoses (2 to the front and 1 to the rear). Since the front calipers hadn't seized, I simply dumped the existing fluid from them and flushed them good with brake fluid. Using a vacuum bleeder, I've gotten the air out of the front calipers but can't seem to get any flow (neither fluid or even air) through the rear lines. I'm beginning to think that replacing the ABS Module and possibly the rear cylinders might do the trick.

With vehicle off pump pedal until it gets hard and yu don't hear any exhaust air coming from power booster . keep foot on pedal start engine . pedal should fall slightly . this is a sign that booster is working . if nothing happens i would do additional test to check power assist booster .

Bleed right rear,left rear, right front, left front in that order to be sure of good bleed but the brake booster makes it easy to push pedal. Make sure of no leaks from master cylinder. Otherwise brake hoses being soft will also hurt baking quickly.