A sealed delivery system for dispensing metered volumes of a pure or sterile flowable substance is presented. Such system comprises a continuously sealing one-way valve assembly at a distal end, and a volume reducing reservoir at a proximal end, in which a pure or sterile flowable substance can be stored. In operation a user pushes down upon an actuator assembly, acting upon a metering pumping chamber in fluid communication with the reservoir via a check valve. Depression of the actuator causes a dispensing pump to push a metered volume of flowable substance through the continuously sealing one way dispensing valve assembly, and return of the actuator to its home state causes the dispensing pump to pull a metered volume of the contained flowable substance from the volume reducing reservoir and through the check valve.