Abstract: An improved supply tank opening cover assembly facilitates excellent fluid transfer between a supply tank and a reservoir, while minimizing leaks. Useful in machines that produce or affix labels, such as franking machines, the preferred embodiment of the assembly includes: an outer cap with an aperture for fluid transfer therethrough; an elastomeric flap placed in the outer cap; a spring member that impinges at one end on the flap, and impinges at the other end on a portion of a support cage; and an inner gasket. With the aid of the spring member, the flap is biased to normally cover the aperture and prevent leakage of the fluid. To allow the transfer of fluid from the supply tank to a reservoir, a reservoir peg is inserted into the aperture and displaces the flap from its sealed position. Liquid transfer from the supply tank to the reservoir is aided by the transposition of air from outside the tank to replace vacated supply tank liquid.