Tracing compressed air system leak

Trying to find a way to find a leak in our compressed air system. Air compressor runs fine and builds pressure in the tank but once the tank outlet valve is open, pressure drops and never builds again. Tried to listen for air leaks, sprayed soapy water on visible connections. No luck.

it s long hose run from the compressor in the ER to the horn solenoid valve in the skylounge, including a second accumulator tank and a drain pipe going back down to the ER…

i considered using pressurized water but draining it all afterward may be be tough and the leak could cause water damage

Has anyone used an automotive smoke generator ? Provides up to 7 PSI or pressure.