There is a circumferential locating groove on the middle cylinder, and the top of the groove and the oil outlet valve seat are sealed by direct plane fitting. The plunger sleeve is provided with oil collecting ring groove and oil return hole. The oil leaked from the clearance of the plunger pair is collected in the oil collecting ring groove of the return oil, and is passed through the low pressure oil chamber of the backflow pump body of the return oil hole to prevent excessive oil from flowing into the oil from the clearance of the plunger pair.