Sounds like you equalization holes in your pistons are too big. 3/4" sprinkler valves should be enough to fire them. 1" would be better. I would suggest twin modified sprinkler valves from www.spudtech.com.

Nice score!
For a load like that I hope there was no "Bad Touch" involved.

It is nice to see people pass on things that can help someone explore advanced ideas. I would not be where I am today if it wasn't for the generosity of people with "junk".

I like the hose clamp idea. Thanks!
I have seen it before but didn't think to use it on this launcher.

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I agree with starman about the zip tie. I can only get the releasable ones in yellow. I will be looking into making a Tekrome™ zip tie in the future. But for now I have to settle for yellow. I will probably try to paint one and see how well it holds up but my guess is that the paint won't bond to th...

The Tek-Valve™ is a 2" piston valve with an integrated pilot valve (no need for an ugly sprinkler valve strapped to the side), gauge, and safety valve. It also uses quick connect tube fittings. The Tekrome™ is a mirror like finish.
I have launched a lot of spuds with a similar launcher.

This is a shirt launcher I custom made for a drag race team. It uses a Tek-Valve™ with 2" porting and has a Tekrome™ finish. It also included a 125psi fixed co2 regulator so it could be used with compressed air or co2. The fittings were painted with truck bed liner. The zip tie is releasable so the ...

I think I understand now. The air bleeds past the piston seal to fill the chamber. In that case It sounds like "your piston isn't a good enough fit" like MrCrowley said and too much air bleeds past the piston when you vent it.

Do you get any air out your barrel when you fire it?

I have some friends that made one. Sorry I have no pic of it. It did generate HHO and they think it will work in a car but I don't think the science supports it. The energy that goes into creating the HHO can be higher than what you get out. You can fry an alternator if you are not careful. If you u...

I must be missing something because I would think you would need an equalization hole so the pressure could get into the chamber.

I try to never go over 120psi with my stuff unless I am trying to blow something up.
You should experiment. Depending on your design you may find a lower pressure that will perform about as good as full pressure and be safer.

Is your piston leaking around the face or the side?

If everything is pressure rated you should be fine. I have not seen any problems.
Just to be safe you may want to try to find some tennis ball lube at your local sports store. You may have to ask be cause they often keep it in back.

More details would be good.
If I'm not misunderstanding your design, it sounds like the hole in the piston is too big and it is just venting the air through it instead of moving the piston.

If your sprinkler valve is not opening your hole in the diaphragm may be too big.

Hope that helps.

Nice cannon! Welcome to spudfiles.
If this is your first I can't wait to see your next ones.
A nice piston valve could clean it up a bit.

If I'm not missing something, I think the piston would oscillate at the outlet too rapidly and not dump either quickly.
It would be cool for me to be proved wrong on this because it would have a lot of potential if it works.
Maybe with a longer heavy piston it would better.

It would need to be something rocket like so you could predict what end would be up when you take the picture/video.