ahh, the fun of "inboard brakes" this brake system will require considerable effort to get into, and pretty much explains your reluctance to have to dig into the brakes.
at this point I would want to insure that there really is enough free play in the mater cylinder rod to be sure the master cylinder is allowing all the brake fluid to return.
Getting into this brake system is not a fun or trivial job, and not a good one for anyone who has not done one before to try without either someone who has done one before, watching to make sure no one gets hurt (and to help "man handle" the assembly which is close to or over 100 pounds) or at the very least has the book to see what they are getting into.
If someone does have a book for this truck, and they would be willing to share the parts about the brakes with you, they will need an e-mail address, and you can not post a properly formated e-mail address on the forum, but you can add it to your profile.