SpudFiles

I am currently trying to build a barrel sealing piston valve for a mini pnumatic. My problem is that the air leaks out of the barrel most of the time. and when it does seal the blo gun does nothing but let the air out. the seal on the pistion is thick rubber from a downhill bike inner tube. the rubber is very soft and has worked in a previous coaxial cannon. the valve body as a 3/4 x 3/4 x 1/2 inch Tee

I don't see a pilot valve hole in your piston, plus the piston should be a cup shape, so as to seal off the release chamber thats behind the piston, that way all the air don't leak out when you pull the trigger, thats what it looks like to me.

Sweet piston, small and compact! If you could provide a little bit more detail on the construction details that would be great (you know the barrel-tee interface and such).

What pressure does it leak at?

Like jrrdw said, its the cup that makes a good seal, however it seems that your barrel and piston are roughly the same diameter so Rather than a cup make it so that it protudes into the barrel and seals.

Do you mean like this?
other wise what else do i need

I think he means it should be more like this. (which it should for better sealing performance.) The cupped piston will move just a bit more to the left as it locks onto the barrel. The piston will actually be slightly inside the barrel.

Thanks Spudstuff I am definately going to make this, and thanks sgort thats exactly what I meant!

I would like to see this gun complete now so be sure to post some good pictures.

dont tHAT DOSEN'T WORK

Huh? The cupped piston thing works for me.

Why doesn't it work? If anything it should work better (if you get the shape right).

will the blowgun exaust a pilot that sise?

When i say cup, i mean the inside of the end cap, should only be in the tee far enough to block off the air chamber, the barrel has to be in the middle of the tee. Then the end cap creates the relief chamber that the blow gun is attached to, but still enough room for air chamber to vent into the barrel once you pull the trigger/vent the relief chamber. There is a more detailed drawing of what i'm talking about in spudwiki, under the piston valve link.

Okay, okay. I left out the tee. Geez. Sorry.