It may be a very simple fix.
Are three places a Qb can leak. If the end cap 0-ring is bad, it leaks at the end cap. If the valve stem is bad, it leaks out the muzzle (when the bolt is closed).
Yours sounds like the 3rd possible leak, the 0-ring that fits around the middle of the valve itself, sealing it to the tube. The valve is in two parts, with the o-ring in the middle. Idea is that when the vale is in place, you tighten it, and that forces the o-ring out to seal the gas tube.
Might be that that o-ring is leaking...or it might be that the valve wasn't tightened enough to force the o-ring to seal.
I'll go look for some pictures.