Dude. The safest way is to cut your barrel off, cut all of your fittings off, and rebuild a new cannon with the left over pipe.

Wow 100 lbs at 1k fps?!

OMG I THINK YOU'RE RIGHT!
There is no rubber on the back of my piston lol, just the end of my deoderant can. I was going to try to describe how the air was getting straight from the chamber to the valve, but decided it was too complicated. I'm, going to modify my piston tomorrow, Thanks for the help.

If it's not "pulling back"... Your piston isn't moving backwards, it is because of a bad pilot or a bad sealing piston... It's not rocket scientist... When a vacuum doesn't pull an object, it's not a strong enough vacuum... I have no idea why you spent $40 on a sprinkler valve, when you can buy one...

It wasnt just the sprinkler valve ($14), I had to buy pvc and copper fittings $12, a blowgun $7, and more pvc glue $6 because I ran out. It all adds up.

Edit: I made some videos to make it a little more clear if you didnt understand the problem before.

DYI wrote:You know, you could have just posted at the start that the reason for the failure was DWV fittings, and saved us about a page of debating.

Yeah, I fixed that earlier today. I really appreciate all the help though.

The first video is all I needed to see. Air isn't getting passed your piston. The ammount of air that comes back out is no way a chamber full. That 1/4" hole should pilot that piston just fine. You need equilization hole/s in your piston. Where exactly would I put the equilization holes? And wait w...

Well if that's the case, I'd say that the problem is just a matter of poor solvent welding technique. Possible causes of failure would be improper primer application, improper glue application, not pushing the fitting all the way into the socket, not twisting the fitting as you push it into the soc...

The first video is all I needed to see. Air isn't getting passed your piston. The ammount of air that comes back out is no way a chamber full. That 1/4" hole should pilot that piston just fine. You need equilization hole/s in your piston. Where exactly would I put the equilization holes? And wait w...

I usually pump it to 60, in that vid it was at 70 I believe. I'm going to buy a 1/2'' ball valve first thing tomorrow.

Actually, I have an extra 1'' sprinkler valve that I modded.

Also, any idea on how to stop the leaks from the screws?

Wait a minute. Let me get this straight... he used my primer and glue to fix his gun but I guess he did it wrong. To fix his gun? He didn't screw up the solvent welding and try again with the same pipe and fittings, did he? Because if he did, the source of your problem has become very clear all of ...

Vids working now.

my pneumatic goes up to 120 psi (never went over that) and my fittings never leak, he used my primer and glue to fix his gun but I guess he did it wrong.

And thanks for the site bob.