SpudFiles

I came up with this when renovating my long-ignored airsoft semi. By cutting grooves in one of the edges that seal against a floating o-ring (described here), I'm allowing the air escape around the o-ring in one direction. When the pressure is reversed, the o-ring seals perfectly against the other, solid edge.
This is ideal for piston valves in cannons that are charged through the pilot area.

The image below is my implementation, it's kind of ghetto but it works great and seems very reliable. I will upgrade my other cannons to this design if/when their current pistons fail.

ya, this is fairly common practice when building pumps and pistons. I did the same thing on my mini pump.

Nice, yes it would be ideal for a pump as well, thanks for the tip. I'm getting into spudgunning again after a hiatus of several years. Too bad there's no good way of summarizing the advancements done in the meantime!

I also saw the same thing in my hand pump for balloons and balls. One side has two grooves cut in it to allow air to go back in front of the seal.