I did some work to the regulator of one of my guns the other day. After putting it back together, I could not get the "release" valve to close, and as you say, pressure just went in from my air cylinder, and right out the barrel.
I tried with the cocking lever closed, I tried with the cocking lever open, but not...in the full cocked position, AND with the cocking lever in the "full cocked" position. Nothing worked.
One of my attempts to fill, I used my thumb to plug the barrel port on the receiver (the barrel was not in place).
As the air went into the guns cylinder and then into the receiver, it was stopped by my thumb. All it took was about 1-1/2 seconds, of this and the release valve closed, and I was able to fully fill the gun with air. No problems have risen from this early leak.
Give it a try, can't hurt and no, it is not dangerous to your thumb.
Have you contacted the seller or Air Gun Technology for help ?
Mike