If my ballpark calculations are correct, you're losing 3.7 gallons of
air (at atmospheric) per hour, so you should be able to see bubbles when
you hit it, unless it's distributed over a lot of joints, each leaking a
little bit.

Or.......

Leaking back thru the checkvalve, into the cylinder and out the
crankcase or intake valve?

Did you spray soap in those areas ??

Grummy