A reciprocating pump with chamber-charging mechanism is provided. In an example diaphragm pump, a discharge valve can be opened by the pressure of fluid leaving the pump and can also be opened by an intermittent mechanical linkage actuated by the reciprocating member powering the pump. The discharge valve is mechanically opened to allow pressure backflow into the pumping chamber, thereby charging compressible fluid mixtures and gases with an increase in pressure. The increased pressure enables the compressible fluids to open the discharge valve on the next compression stroke and exit the pump. In an implementation, the discharge valve is pushed open by the reciprocating power source in a configuration that seals the valve mechanism from well fluid. In another implementation, the discharge valve is pulled open to pre-charge the pumping chamber.