Hook a scanner up to it and go thru the abs bleed proceedure on it. Do this several times switching off vehicle and then restarting the proceedure again. Then go the r/r, the l/r, the r/f, then the l/f wheel and bleed to all you see is straight clear fluid. Al the while making dang sure the M/C never gets low on fluid. (I did this same deal at the shop.., drove me crazy trying to get barakes on that ford. Finally and I mean finally, after almost a gallon of fluid.., the pedal started to feel good again. I was a happy camper to see that pos leave that day!!)
Usually, and I said USUALLY, the air will get trapped near the rear.. specifically the left rear wheel.
I learned this the hard way.., either to vise-grip the rubber hoses(not enough to kill the hose.., just enuff pressure to close it off) or find a junker and cut the metal lines that go to the hoses.., beat the line part flat and then fold the line up and over itself 3-4 times.
Keep these in your toolbox for having to do stuff like this. Or find some kind of plug that will stop the brake fluid from coming out of that line.
All in all.., never let an ABS system go dry or close to dry. It will become a frequent headache and drive you insane when trying to bleed it out.
Another option..., have someone pump it, then attempt to bleed the abs one line at a time. (harder to do than stated.., but it will give you a 75% chance of getting the air out.)
Lastly.., if your truck has drum brakes on the rear.., make sure the shoes are adjusted correctly. If they are very loose.., this can also cause the "no pedal" or spongy pedal effect as well.
(Due to post of loosing pressure after 30 sec, this was added)
I would also check into the Master Cylinder as well. I've seen new ones that got run dry for a few and they ended up being replaced after that. I would also try bleeding the M/C and make sure it has really good pressure coming out of BOTH outlet ports. If not, take it back off and try to bench bleed it to see if it can get Good pressure out those ports. If still not.., then replace it. Make sure to bench bleed the replacement before you install it back onto truck. Then comes the "bleed everything" fun. Start with M/C..again. Then use the scanner to redo the bleed proceedure again.., then R/R, L/R, R/F, And finally L/F wheel again.
I sure hope you can get it straightened out. It sure sounds like it has been a total PITA deal for you from what I read in your previous posts.