Air leak

well it will drop the air some when you pump brakes, if it drops too much when you apply brakes then doesn't drop anymore, typically that means your brakes ned adjusting. if you apply brakes and it keeps going down, then you have a leak on the service side somewhere. your leak sounds like it may be on the emergency side.

 

You haven’t stated whether your air tank gauges drop before compressor kicks in.. if they do you have a major leak when running (fan hub?) brakes etc .. if gauge doesn’t drop much or any I’d look at the signal line to the governer

 

Had the governor sensor line cause that, had a loose push to connect fitting.
Only leaked when engine running, compressor would kick back in within 15 seconds of purge.

 

Fan hub.

 

The fan hubs can leak a lot and are hard to hear. Yours needs air to keep the hub disengaged. This only happens when key is in run position or engine is running. With truck fully aired up, shut truck off then leave key in run position. Your fan should free wheel now. Listen closely as possible to the hub. Should be able to hear it

 

Not that I know of. Also not a leak that is dependent on engine running or not.