Abstract: A pump for pumping high viscosity lubricants such as wheel bearing grease comprising a pump tube adapted to be disposed within a container of the lubricant, a piston rod and piston movable within the tube, an outlet at the upper end of the tube, an inlet at the lower end, a check valve in the piston and a valve at the inlet. Upon the piston downstroke, the inlet is closed by the valve and lubricant is transferred through the check valve in the piston from below to above the piston. Upon the piston upstroke, lubricant above the piston is forced out the outlet and lubricant is drawn into the tube below the piston through the then open inlet. The inlet valve is constructed so that it cooperates with the piston to develop a vacuum at the inlet before the inlet is opened. Upon opening the inlet, there is then a sudden exposure of the lubricant surrounding the inlet to the vacuum which draws even high viscosity lubricant into the tube to prime it. Reliable pump priming is thus achieved.