A main valve diaphragm closes the liquid inlet to the tank when the latter is full because ballast members associated with a pilot valve have their gravitational effects negated as a result of liquid in the tank being above or at substantially the same level as such members, thereby allowing the buoyancy of a float to maintain the pilot valve closed. On the other hand, when the liquid in the tank drops to a predetermined level, the ballast members become effective to gravitationally operate the pilot valve which in turn releases hydraulic pressure in a chamber above the diaphragm that otherwise keeps the diaphragm seated. The pair of ballast members on the exterior of the assembly are each independently adjustable into any one of a number of selected vertical positions along the assembly, the position of the upper member determining the location of the upper level of liquid in the tank, while the position of the lower ballast member determines how far the liquid level will drop before the valve diaphragm is unseated to allow the entry of a new supply. All components of the assembly are designed for installation without the aids of tools. Alternative versions of atmospheric breaks render the assembly anti-siphon.