1989 Ford F350 Can't get hard pedal on brakes

I have some brakes but unsafe to run on the road.
Brakes go clear to floor. When bleeding, air blasts out right rear (longest line); after a few procedures, it runs clear. Did all four four wheels in proper sequence and even bleed the master cyl by loosening connections. Pressure seems less with engine running.

I'm totaly baffled. Could it be sucking air somewhere? I checked the entire system for leaks.

Yes, air seems to be creeping back especially the right rear (the longest line). I would bleed them 4-5 times til finally no air, then go around to the remaining 3 wheels only to go back again to the right rear and find air again. I would have my buddy stand on the pedal, trying to bust out a line if he could to find the possibilty of a small leak somewhere--no find.

Another bit of info--I mentioned earlier the proportioning valve was by-passed with a manual valve. This was done at a brake shop, but the tech did not disconnect the wires going to the proportioning valve. I was hopoeful after disconnecting the wire would solve my problem but didnot.

I also saw on a Ford forum website where someone described my problem to a t, saying it was solved by "cross bleeding" but didnot describe how that was done. Another individual said the hydraulic hose at the fron wheel from frame to claipier deteriotes somehow to form an internal flapper vlave. He said after changing the hose, it solved the problem. I changed one hose but didn't change the passenger side yet. I appreciate any suggestions you might have. Thanks!