I have the MBRP DP. I dont believe mine is leaking but not sure how to check without pressurizing the whole system. I do have a ticking sound but from what i've read that is pretty common on these pickups (believe there is even a TSB on it). Its louder on the driver side but its certainly not a hissing sound at all.

Are you sure you tightened the v-band good? if the flanges mate up nicely then the only way i can think of it leaking is large pressure finding the slightest exfil route (thus the hissing) or it un-mating and causing the leak.
Given you've went to the trouble to build a pressure test system to find it... normal logic would tell me you've already tightened and re-tightened the clamp.
With that assumption, the only thing else to think of is whether or not the vband is positioned correctly. Unless you pulled the cab, accessing that clamp is NOT easy. I remember doing it myself and i had my brother help me to ensure it was aligned.

 

Thanks for the response! I've been in and out several times. I finally bucked up and ordered and MBRP as well as another new clamp. Well see if this will do the trick. Should he here on Friday.