Near the base, the pump communicates with the outside through two small pipes, each of which equipped with a cock and a valve held shut by a light spring; of the two pipes, one is for sucking up the air or gas to be compressed and the other to deliver it to its recipient.
The valve in the suction pipe opens from the outside to the inside while that of compression opens from the inside to the outside.
Also in this kind of pump there is a clearance volume which is delimited by the base of the piston when it is at its lowest position and the base of the pump body to the two valves. There is thus a limit on the maximum pressure that can be reached in the recipient which, if the opening of the valve were automatic, would be given by the pressure reached in the pump body after a certain number of suction and compression cycles when the air or gas drawn in and then compressed occupies the minimum volume available, which is that of the clearance volume.
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