73 GMC C2500 Right Front Brake Not Working

[Martin64]

My sons trucks brakes haven't worked well since we bought it a year or so ago. The rear brakes would lock and nothing (or nearly nothing) on the front. Bled the brakes, checked the shoes. Worked ok for about a week and now back to zero. Just changed front calipers thinking piston seals were bad. Back to zero. No brake pressure getting to passenger side front caliper. Could not get fluid out of the bleed valve.
Drivers side has plenty of pressure. When we bled it, it had full flow and great pressure.

Any thoughts?