A pumping device having a bellow-type pumping assembly having a stationary and an oscillating end portion, and a mechanism for reciprocating said end portion between a given minimum volume and an adjustable maximum volume position, the said mechanism including a spring biased connecting rod, designed to transmit a force not greater than a given value, and a motor reciprocated arm along which one end of said rod is pivotally connected at operator adjustable positions for modifying the actual length of said arm and therefore the amplitude of the variation of said volume, while the pressure built-up in said chamber cannot be higher than that provided by the force which can be applied to said oscillating end portion by said spring biased rod.