SpudFiles

http://smg.photobucket.com/albums/v611/ ... =valve.png

That's a 3/4" tee, pipes leading into the top and left of the fitting are 3/4" sch 40 PVC, pipe to the right is 1" sch40 PVC - and would be longer than is pictured.

Ok, it's a slightly atypical barrel sealing piston valve built into a 3/4" tee. Porting is 3/4", piston moves all the way out of the way for improved flow.
Aesthetically, it's small and sleek.

GGDT shows that it works quite dandy with a blowgun as a pilot (works well with very small chamber volumes)

Only two machined parts required; I've included .PNGs and .DWGs of these to make it easier for you to make them/have someone make them for you.
You may have to change the length of the piston depending on your brand of tee... I believe this is based off the dimensions of a Lasko fitting.

Piston is CPVC for improved impact strength and machinability.

Valve is assembled using epoxy because solvent-welding tends to change part dimensions.

Feel free to suggest modifications.

I think the only modification I could possibly add to the design is to rename it a chamber sealing piston valve

i think you forgot the equalization hole

Ah yes, that too

Re. E.Q. Hole. Since it's at a right angle, is'nt that gonna be kind of a bitch to drill?

There is a equalization hole; it's the green hole labeled "[diameter] .031"

It's hard to see in the frontal view, because the cross hairs obscure it.

And yeah, I'm being a very bad drafter and dimensioning a hidden line. I was lazy and didn't want to draw up an entire new view just so I could label the stinking 1/32" hole.
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And no, it should be easy to drill:
1) Precision placement isn't required
2) You can just drill it in from the rear of the piston, which should be quite easy.
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And, eh, I'm pretty sure that this is a barrel sealing valve.

However, if one where to drill the EQ hole down the center of the piston (which would require a 1/32" bit a good deal longer than "jobbers"), it could be used in a chamber-sealing fashion.