Abstract: A modular hollow sliding piston fluid valve, includes at least one inlet port and at least one outlet port both attached to a housing, and an axially-slidable piston inside the housing provided with means to close an inlet port when in proximity thereto, the piston being configured with at least one opening allowing fluid to enter the inner part of the piston from the inlet port and flow through the opposite open portion of the piston and through the outlet port when the piston is moved in proximity to the outlet valve, the housing having provision for the possible addition of a third port disposed at about 90° to the axis of the piston.