Pedal goes to the floor... And i am stumped

Ragtop, this may sound stupid but does the truck have abs brakes? If so I have seen this happen on a couple of other makes ie. ford . But the abs system had a dump valve that opens during antilock situations. It sounds like you've got everything else correct. I don't know these vehicles that well if it has an abs controller of some type, try unplugging it, then see what happens. Hope this might help!