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that it dosent escape back past the seals then how does the next 'dose" of air get past the o rings when the pump is withdrawn.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