Sounds like it could be a proportioning valve.
Possible. Not likely.
Possibly collapsed rear soft line.
Likely. I've had multiple OEM-rubber brake hoses on various vehicles that plugged up tighter than a bull's asss in fly season. NO amount of standing on the brake pedal would push fluid through those hoses. Brake pedal was
high and firm (almost hard) and there was surprisingly little braking power.
It's not that the hose "collapses", the rubber swells and closes-off the fluid path.
At some point between "new" and "plugged", they'd merely be "restricted".
Other possibilities are that the metal brake tubing is dented-shut (or nearly shut) somewhere; and the RWAL isolation valve is stuck shut. RWAL has a bad reputation on this forum, but if that truck were in MY driveway, I'd (carefully) unhook the rear brake tube from the hose at the frame bracket, and see how much fluid was available during bleeding. Lots of fluid there almost certainly means a plugged hose.
Or if the master wasn’t bled properly there could be air in the rear valve for the master. Did you bench bleed it and cap the ports before putting it on?
Either the air would bleed out, or the air would make no difference in the fluid flow. Air in the system isn't blocking the flow of fluid. Trapped air
could cause the pedal to be "super spongy", though.
If you think it is the valve you can probably check it as follows and it is only meant to be what i would try.
put the parking brake off then turn the key on and wait a second, make sure the brake warning light goes off after a few seconds (park brake on and the light will stay on). if the warning light stays on, go unplug the brake warning light connector on the valve and then go an make sure the light went off. In this case it is the combination valve. Just to be sure, with the connector off get a multimeter with continuity and touch one probe to connector pin (on the valve not the harness) and the other to anything connected to ground, the side of the truck, a tube, whatever. if there is continuity then the brake warning spool has shifted over and could be blocking flow to the rear.
I see this on multiple forums; and I don't know where it comes from.
The safety valve / spool
doesn't block fluid flow.
MOST if not all GM safety valves are
self-resetting. They're spring-loaded to the center position, they only go off-center and turn on the dash warning light when there's actual pressure difference between the two master cylinder hydraulic circuits.
I have dicked with FORD safety valves (twice) that had to be manually re-set. Ford's "better idea", I guess. Foookin' pain in the tuckus. Hateful job. However, even the Ford safety valves, when off-center, don't block fluid flow.