Quote Originally Posted by robs5230 View Post
On opening the pump arm, there's a point at which the vacuum in front of the pump seal will vent, either by the seal clearing a slot or a drilled hole, allowing the next lot of air forward of the seal to be compressed on closing
Quote Originally Posted by T 20 View Post
There's either a vent hole at the end of the pump stroke or the piston head moves forward far enough so that it breaks cover in the cocking lever slot to take in air.

The vent hole in the cylinder is small enough and finished well enough that it causes no wear on the seal.

I've never seen a multi pump with a one way valve in the piston head to allow air in, it would just be an over complication that's not needed.



All the best Mick
Yep, absolutely.

And, although unlikely to ever cause seal wear issues, the hole or slot could be de-burred if considered necessary.