Gas powered.
Proportioner.
Been using for 3 seasons.
Pressure is fine after startup.
Turn on soap, pressure drops, pulsates.
Turn off soap, pressure sometimes returns, sometimes keeps pulsating, about every 3 sec intervals.
Turning unloader on/off with gun on sometimes resolves.
AG hose is vibrating so I think there is air in the line. Not sure where air is getting in.
Been dealing with issue from the end of last year.
If the check valve cleaning doesn’t do it, easy enough to just prime one line at a time to narrow it down, which it seems you already know where your problem is. Good luck
In addition to the check valves, I’d also check for any loose hose clamps, cracked filter housing and input lines that may be getting worn on the bottom where it lays on floor. If you are using rubber hose for input, check to make sure the inner layer hasn’t collapsed causing restriction. Long time chemical exposure can soften that interior lining.