Stop in for a cup of coffee

Service call. Buddy has been battling with his all new brake system in the 50’s chevy truck hes building. Fluid to back, very little to front calipers. Checked prop valve, its flowing. And wouldnt gravity bleed at all.
After almost 2 hours of poking and prodding, it came down to a loose fitting. Right at master cylinder. He snugged fittings, but the flare didnt seat or seal. I saw a drop come from the line.one drop. It was sucking air back up into master. I spent 2 hours,he’s been fighting for 3 days. Im happy i solved it.