Gas Roof Pump Question

I have a 6 gpm gas roof pump A/R 125. It will build pressure, have it set to max out at beginning of yellow zone maybe 200 psi. When I hit the trigger it drops to about 50 psi but will not build pressure from there. Max distance about 22-25’ feet in past it would hold at 150 hitting 35" + Any suggestions on how to get pressure to build ? Every thing is tight not getting air in system

still sounds like air. Have you recently done anything service to the pump, lines or proportioner or anything?

Other than tightening thins up how do find where air is entering system?