First, spray a shot load of silicone spray into the valve. If that doesn't work...
Second, you're going to have to open your take. Cut it at the ridge where the tank gets wider at the base. Make a straight a cut as possible. Before cutting, draw a line down the tank, so you can line it up later. Now fuck around with the tank, cover everything in silicone grease, and glue on the piece that keeps popping off. Try to fix it anyway you see fit. But I don't think you can replace that piece. It's custom made for the tank, plus it has a different density than most other rubber fender washers. Epoxy everything back up with PLASTIC SPECIFIC epoxy. If it still doesn't work then replace it with an AT2K tank.
If you screw up your tank cutting it open don't blame me. I've never tried it, but I've seen it done to a 2K with success. If you follow my instructions all SHOULD go well.
Also, shoot a PM to imaseoulman. He has extensive experience with 1.5K's.
Ok thanks. Yeh I actually cut the the tank open exactly where you mentioned and I glued the piece better and cleaned up anything that might have been causing the leak, but when I glued the tank back together, I still had a small leak. Although no damage caused by cutting the tank open fortunately But if need be, I can cut it open again. The gun works fine if you shoot right after you pump, but if you were to wait say 10 seconds before you shoot, it doesnt have much velocity.