He means the one in front of the threads. If it's not sealing, you'll get a leak out the back. If I understood correctly, your leak is coming out of the bolt guide's set screw, the one used to seal off the HP feed to the valve chamber, right?
If it's both (try not to use teflon tape), you should remove the cap's set screw (#7 in the sticky's diagram) and apply blue loctite, then screw it back in, then change the o'ring in front of the threads (#4 on the right on the sticky's diagram) and lube it with dow55. That should fix the leak on both locations.