Inlet valves, in a fluid-pressuring cylinder, are biasingly held in given operative positions where they occlude passageways, formed in the cylinder, which open at opposite ends onto the cylinder interior and the cylinder exterior. When an incompressible slug of liquid enters the cylinder, the relevant valve retracts against the bias thereof to expose the passageway, whereby the risk of over-pressuring is avoided, and the excess pressure is simply vented through the passageway(s), directly to the exterior of the cylinder.