A mechanical fluid pressure modification valve, or "MFLP-valve", is set forth. The MFLP-valve can be actuated upon movement of a movable pressure system member of a medical pressure system reaching a selected position. Additionally, the MFLP-valve can be actuated based on a pressure condition of the system. When the MFLP-valve unseals, a pressure differential between a pressure chamber and a volume outside of the chamber can be relieved. Disengagement of the movable pressure system member from the MFLP-valve can enable the MFLP-valve to re-seal. The MFLP-valve may include a flag that indicates the position of the movable pressure system member. The MFLP-valve may be provided in a diaphragm-type pressure source.
