SpudFiles

I had the exact same problem on my co-axial. I found some dowel, well a broomstick that fit perfect cut a bit off. Then with a dremel (you could use a file) made an o-ring groove.

Which was actually really easy with wood, it wasn't perfectly straight but it was pretty good. Then I chucked on an o-ring, lubed it up and it worked perfectly.

I think the problem was, although the first piston was a good fit, it wasn't a good enough fit to be piloted with such a small chamber.

ok cheers mate ill give it a try i have an o-ring from the same bike pump i got the first piston from so ill use that

Just use a bigger pilot valve. i couldnt get mine to fire until i used a ball valve.