A well pump assembly (24) has a barrel 54), a standing valve (48) at an upper end of a standing valve chamber (46), and a plunger (70). A travelling valve (82) admits well fluid into the barrel during a fill stroke. The travelling valve closes during a power stroke so that the plunger pushes well fluid from the barrel into the standing valve chamber. A gas release port (88) extends from the standing valve chamber to the exterior of the pump assembly. A check valve (90) in the gas release port has an outward flow blocking position for blocking liquid well fluid in the standing valve chamber from exiting through the gas release port while the plunger is in the power stroke. The check valve has a gas release position that enables gas present in the standing valve chamber to flow out the gas release port while the plunger is in the power stroke.