do you have some pics of your system? far enough away so we can see what you have to work with?
-pedal style, linkage etc to booster
-booster and master cyl style and mounting location
-any res valves in line
-line connections at master and to prop/combi valve
-a full shot of the truck, it'd be cool to see what you are working on.
here is a quick video (the second link) on how the prop/combi valve operates. it seems like if you had that much pressure applied to your pedal and still have no real brake application to the rear circuit, then maybe the combi valve is locking out the rear circuit due to having sensed a large pressure differential between front and rear circuits while you were bleeding the system. maybe it just needs to be reset to zero again so the piston in the middle (pressure differential valve) is centered. when you bled the brakes did you use the tool to prevent that from happening, where you unscrew the switch and screw in the tool? that's why I asked in a post above if the switch were "turned on" which would indicate the piston isn't where it should be and the rear circuits may be simply locked out. you can easily reset that piston in the pressure differential valve by unscrewing the switch and moving the piston back to center with a clean pointy tool, like a small screw driver, an awl, a long nail etc, something that fits into the small hole at the bottom of the switch etc. use something that isn't painted or that can't flake off a coating when the tool touches the hole or the piston. simply move the piston to center using a prying action, install the switch again and then try your brakes. it might be that simple.
check these videos out for an explanation with cutaways of how the combi valves work and how to tell at a glance if you have the correct valve for a disc/disc system (look at the front of the valve where the front brake lines hook up). the first video shows the differential valve and tool at about the 3 min mark. it explains about the piston and switch I am referring to and also the tool that keeps the valve centered when bleeding. the second is just a video showing the different valves available and what they are for/how they work.
https://www.youtube.com/watch?v=3z1q7MVBs08
https://www.youtube.com/watch?v=dD1dbNtH-6s
https://www.youtube.com/watch?v=qPLaPv8DITE