[Jimski]

I discovered a leak in my hop hup assembly precisely by putting it in a water tank and blowing in it.
It was a very tiny bit of plastic that got broken, I fixed it with teflon tape.

I also had a problem on a piston head once, specifically the Oring was dead/too small; it Would completely loose compression once every 4 or 5 shots, and the noise is VERY caracteristic and easy to tell.