one of the housing portions to move in one or more other directions or axes without leaking or breaking prime thereby providing an expansion or contraction in housing volume in response to pressure inside the housing the bias provides a restoring force that allows the expansion joint to be self resetting alleviating a pressure increase can be sufficient to prevent freeze expansion damage to the pump and or can be sufficient to reduce pressure fluctuations in the pumped liquid such as at the outlet of the pump alleviation of pressure fluctuations is further facilitated by the ability of the movable portion of the pump housing to exhibit a volumetric contraction when subjected to a pressure decrease in the housing according to the present embodiment volumetric and hence pressure compensation is achieved by the housing itself correspondingly changing the area of at least one of its pressure boundary walls or portion thereof to illustrate consider a pump housing such as any of the housings in the embodiments described above the wall in substantially any part of the housing represents a pressure boundary and hence is a pressure boundary wall if there were no pressure difference across the wall there would be little to no pumping action produced by the.