Did that, still has same problem.

Sorry about my confusing post, I've never done good in english class!
Everything is cleaned and lubed, no shaving or anything on the diaphram or elsewhere. And it isn't put on backwards. Obviously the diaphram isn't sealing, but I can't seem to find out why..

Hey guys, I just recently built a small pneumatic spud gun, both the barrel and chamber are 18" long. I modded a sprinkler valve according to the 'official how to' posted on this site. I used the same base sprinkler valve as the one I use for my longer version that has an 45" chamber, a Orbit Waterm...