The cryogenic pump is of the reciprocating type using a differential diameter piston assembly having a reciprocating piston and piston rod in which the piston is of a diameter larger than the piston rod so as to form a variable volume annulus within the pumping chamber about the piston rod. The pumping chamber is surrounded with a cooling jacket connected by a passageway to the variable volume annulus. Blow by fluid is collected in the variable volume annulus during each suction stroke of the reciprocating piston and passed into the cooling jacket during each discharge stroke so as to cause collected blow by liquid in the cooling jacket to flash during consecutive discharge strokes.