LP System pressure is ONLY 0.39 psi downstream of the Regulator
If you tried blowing air into thru line:
1) Is has no place to go unless you disconnected a line/fitting somewhere on the supply line
2) Propane appliance gas vlaves are NOT to be subjected to pressures above 0.5 psi====can damage them
As for that LP issue
I would start with NEW Pigtail hoses (connect cylinders to regulator) and a New Auto-changeover regulator
Pigtails are subjected to full cylinder pressures and can fail internally
Regulator diaphragms weaken/fail/leak as they age