Quote Originally Posted by trajectory View Post
Thanks for the reply, I dont think I asked the question very well when I posted it so sorry for any confusion caused on my part.
I understand that the air compressed ahead of the piston passes through a one way valve & is trapped/ stored until either another dose of air is pumped into it or vented on firing. What I am trying to understand is how the air gets ahead of the piston for subsequent pumps. If the seal is good enough to trap the air ahead of the piston for the first stroke, compressing it & forcing it through a valve how does the air get ahead of the piston for subsequent pumps? If the seal is that good in the compression stroke to generate high pressure ahead of the piston then I can't see how the air from 'outside' gets past the same seals for the next compression stroke. Maybe I'm not expressing it very well but at the moment i can't see how the pump seals work to allow compression in one direction ie when the piston moves forward to compress the air but when the piston is drawn back in readiness for the next compressive stroke how the air gets into the cylinder ahead of the piston. On compression there is little air left ahead of the piston, it's gone through the valve, so moving the piston back must cause a drop in pressure ahead of the piston, not a vacuum but the pressure will be lower. Does air then flow into it from the atmosphere? If so how can it get past the seals that are good enough to compress it beyond normal atmospheric pressure? I can see how a cup seal can do this but unless I'm missing something I can't see how an o ring can compress in one direction but allow air past it in the other direction. Hence the speculation that there is a vent of some sort or another that allows air in ahead of the piston that gets closed off as the piston plus seals ride over it on the compression stroke.
Does that make sense or have I stirred a muddy puddle?
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