The valve sealing system for a reagent package includes an annular cylindrical gasket seated in a channel formed in the package. A rocker valve on the package pivots relative to the annular cylindrical gasket which surrounds an opening to the chamber of the reagent package. The annular channel includes a radially inner peripheral channel sidewall, a radially outer peripheral channel sidewall, and a bottom portion. An inclined peripheral wall portion of the channel intersects one of the channel sidewalls at an obtuse angle and the bottom portion of the channel. When the rocker valve is in a valve closed position it engages the annular cylindrical gasket to provide a substantially leak-tight seal.