Abstract: A modular valve assembly provides an airtight, shockproof, leakproof and evaporation proof closure effective for sealing engagement with the neck or outlet of a flexible or reducible container. The modular valve assembly is effective in preventing: (1) leakage of fluids from the container due to vibration or changes in temperature or pressure; (2) any backflow or reentry of contaminants through the valve assembly, including air; and (3) evaporation of fluid from the container. If the fluid initially is sterile, the closure maintains the sterility of the remaining fluid in the container during and between dispensings of the fluid. Thus, the modular valve assembly extends the useful life of the fluid in the container to its shelf life. Although the user makes many dispensings from it, the container behaves as though she or he had never opened it. Thermostable fluids delivered through this modular valve assembly have no need for preservatives or refrigeration.