SpudFiles

I've had my coaxial air gun for a while and it never had a problem. Just recently I decided to make and use a 1'' barrel in it instead of my 3/4'' and everything worked perfectly for a couple of shots, then it started to act funny. At 20 PSI it works nice and smoothly (or so it seems); at 40 PSI nothing happens, I'll release the blowgun attached to the modified sprinkler valve and all the air will rush out of the pilot valve, then a second or so later I'll hear the piston succumb to gravity and drop; and then at 60 PSI it is pretty skittish, I'll pull the trigger and the pilot valve and after an unusual delay the piston will strike back and some remaining air will come out of the barrel.
I've tried taking apart and testing my sprinkler valve and blow gun but they worked fine. Then I took out the piston and replaced the duct tape, refit it to be sure there were no gaps but it still moved freely, and re-greased it. Not a thing changed. I even resorted back to my 3/4'' barrel but for every consecutive shot it still had the exact same problems.
Any other ideas or advice?

PS-I'll get some pictures of the piston up later, but all it is is a bolt with (in order) a rubber washer, 3 2'' OD washers, a hex nut, then 2-3cm down another 2 hex nuts with about 6 2'' OD washers sandwiched in between. On top of the washer groupings I have a thin layer of duct tape (I used the outer washers to sandwich the ends of the duct tape in so it wouldn't come off)

I don't like wrapping pistons with tape. Air can get under it and create odd tensions / side pressures causing tilts and drags that interfere with accuaction. I know it has been used with success many times but there is more room for problems then realised. If it were me I would line the cylinder wall with aluminum can to take up the tolerances and not wrap the piston.

Find a friendly machinist nearby you will quickly lathe you a delrin or UHMWPE piston

Check your sprinklervalve.
Sounds like the diaphragm is the problem.
Maybe it's damaged or not installed right.

Similar problem, similar results...just in a different valve?

I have shop class again next year, so hopefully my teacher will let me make two nice new pistons for my air guns (and maybe a third for a later build ). But it turns out that the high density foam I was using as a backstop was clogging up the area between the pilot valve and the pilot area. Works beautifully now. Thanks for all of your replies!