You need to be done with this job.
Buy, rent, borrow, a little/or big compressor and pipe it up to the right rear Gladhand, Just make some fittings, they can even leak, who cares. Only need 120 PSI maximum, watch your dash gauge!
Go and operate the brakes as much as you want, Crawl all around under truck, do the work and Have soldier B pump the pedal. Do that with wheels chocked really good. Do NOT run the engine and put Everything in Neutral.
Is the vent line atop the master cylinder intact and where is it routed to?
A brand new Deuce master cylinder will survive for about 10 to 150 pumps if its vent is plugged. Is that VENT CLOSED off by bending or whatever?
Does the MC actually have as designed travel. Is the pushrod proper, did anybody mess with it?
Is there any fluid at the forward end, under the boot, of the MC?
This has gone on long enough.
DO you have a remote reservoir and if so did you delete the swashplate?
Are the holes in bottom of MC clear?