How does this work?

[trajectory]

Multi pump air rifles or pistols with a cup washer on the piston.......I get how the air can get past the seal on the priming stroke & become "trapped' so that it gets compressed as the pump is closed with the cup seal stopping the air getting past it if all is well.

But how does it all work if the cup seal is replaced with an o ring or a pair of o rings? If the o ring seals against the inner wall of the pump tube so that the seal is sufficient so that on the compression stroke the air is compressed ahead of the piston & the seal is so good that it dosent escape back past the seals then how does the next 'dose" of air get past the o rings when the pump is withdrawn. If the answer is that it gets ahead of the piston due to atmospheric pressure trying to equalise the pressure then when the pumps operated ie closed again the pressure ahead of the piston will rise & will either go into the valve if the pressure is low but if it's already at 1 atmosphere what stops it escaping past the o ring back the way it came in? Does this mean you can't pump up a pneumatic beyond 1 atmosphere? Not the case.
If the air gets ahead of the piston o rings because the o rings come back over a vent hole of some sort then the air could get ahed of the piston but then wouldn't the o rings get damaged very quickly in use?

Can someone throw some light on this please, what actually happens in practice. I'm sure multi pumps go beyond 1 atmosphere operating pressure, but can't see how the seals would stand up to repeatedly going over an inlet hole even if radiused, polished & lubricated,

Thanks in advance

[TonyL]

Not sure if I'm understanding you correctly, please excuse me if I'm not.

On each successive forward (priming stroke) the compressed air ahead of the pump washer / seal is forced into the intake of the valve. Valve closed by spring & seal. On each successive priming sequence, the valve is forced open again etc etc. Air stored in the valve until exhaust valve stem is struck by the hammer, releasing the stored air through transfer port into the barrel (on most designs; some differ).

[trajectory]

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?

[robs5230]

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

[trajectory]

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

Thanks, I thought there would have to be a vent....those seals must be pretty special to cope with running repeatedly over it, must be very resilient even running over a smooth apperture,

[Solvo]

Thanks, I thought there would have to be a vent....those seals must be pretty special to cope with running repeatedly over it, must be very resilient even running over a smooth apperture,

Why would the vent be in a part of the cylinder that the seals pass over?
The output vent to the storage is most likely in the end, I dare say the intake is likely piped to that same end, or possibly even just a valve in the piston itself. It doesn't have to be in the sidewall of the cylinder, there are other options!