Ok here goes throwing some things at the wall....
Some brand new bottles can sit around in storage and the valve get sticky. Is it a little hiss or gush?
I had a new old stock BSA bottle give a little hiss due to sticky valve, so just exchanged it for another, problem fixed.
Is the pepper pot on the Huma screwed all the way in?
Is the carbon bottle neck more suitable for original reg, not the Huma, rubber pad in wsy.
Did you measure Huma rod to match original reg rod length?
Have you tightened the Huma up as much as possible, gave you left original reg support screw in?
I removed support screw, but did use a little square of chamious leather as a shim, making sure it doesn't block Huma breath hole.
Just a few things for now.
Had mine out this morning bagging a few quarry, stick with it it's worth it.