If it is leaking at the QEV you need to teflon the threads as Proto suggested.
You can also try these troubleshooting methods.
Check these o-rings for damage; replace if necessary.
bolt sail o-ring (16x2-mm ID)
bolt stop o-rings (14/70 and 20/70)
body breech seal (17/70)
Verify that the tip of the bolt and the middle section of the bolt are both unscratched. If there is a large enough scratch, a leak may result.
Verify that pressure is indeed moving through the solenoid while the marker is idle. For this, you will have to first separate the frame from the body and remove the banjo fitting from the vertical adapter. Then, pull out the circuit board so it is free from the frame, and screw the banjo fitting back into the top of the vertical adapter. This will allow you to test the marker.
Remove the middle banjo fitting from the underside of the body breech (the one with the 1/8" microline). Now, take an object and stick it inside the feedneck to prevent the bolt from closing (this won't cause any damage). When you now pressurize the marker, pressure should continuously leak out the 1/8" microline. If air doesn't leak, then the solenoid is the problem, and should be disassembled and cleaned, and reassembled correctly. You may be able to hold your finger over the middle banjo for a short period of time, to keep air from rushing out.
If air is indeed released from the microline in the above step, your solenoid or board may be damaged, causing the solenoid to fire continuously. If this is the case you'll need to replace the board/solenoid.