Originally Posted by Smiley
The lower tube to valve chamber solder may have flowed away from the 10 x 32 threads at the chamber inlet. The oring in the banjo fitting only seals to the outer surface of the lower tube, pressure may be leaking between the valve chamber and the lower tube to get to the front grip screw threads. Try spotfacing the sealing surface a little deeper so the oring seals directly to the valve chamber wall.
good point smiley, thinking about it, maybe a couple wraps of teflon tape might seal the gap? most of it will be shed in the first few threads, but it's worth a try for an easier fix or at least to help find the leak...