A high pressure pump assembly comprising a pump housing and a driveshaft which is rotatable within the pump housing, the driveshaft comprising a front journal portion, a rear journal portion, and a cam portion, located between the front journal portion and the rear journal portion and within a cambox of the pump housing; wherein rotation of the driveshaft causes the cam portion to impart a force upon and cause reciprocating motion of a pumping element of at least one pump head (112, 114); wherein the driveshaft is supported in the pump housing by at least one bearing (140, 142, 144) located over a journal surface (124, 126, 128) of the driveshaft; whereby the pump housing comprises a chamber adjacent to an end of the driveshaft, wherein the chamber is supplied with fuel from an inlet which is supplied by a fuel tank, and wherein the chamber is in fluid communication with at least one bearing (140, 142, 144) and at least one journal surface (124, 126, 128), such that in use of the pump assembly, fuel is forced from the chamber to a backleak path, via the at least one bearing (140, 142, 144) and journal surface (124, 126, 128), wherein the chamber is in fluid communication with a rear bearing and a rear journal surface over which the rear bearing is located; and wherein the chamber is also in fluid communication, via drillings (134, 136, 138) provided in the driveshaft, with a front bearing and a front journal surface 126 of the front journal portion 120 over which the front bearing is located, and/or with a rider bearing and a rider journal surface of the cam portion over which the rider bearing is located, wherein the drillings (134, 136, 138) provided in the driveshaft comprise an axial drilling, a first radial drilling provided in the cam portion, and a second radial drilling provided in the front journal portion; wherein the axial drilling communicates the first radial drilling and the second radial drilling with the chamber; the first radial drilling communicates the axial drilling with the rider bearing and the rider journal surface; and the second radial drilling communicates the axial drilling with the front bearing and the front journal surface, wherein fuel flowing from the chamber via the rear bearing and rear journal surface, and via the rider bearing and rider journal surface, flows into a volume within the cambox, and wherein fuel flowing from the chamber via the front bearing and front journal surface flows either into the volume within the cambox or into an annular gallery provided within the pump housing around the driveshaft; and wherein the backleak path comprises: a first exit pathway section providing unrestricted fluid communication between the volume within the cambox and a common backleak pathway section; characterised in that a second exit pathway section, providing fluid communication between the gallery and the common backleak pathway section, wherein the second exit pathway section is provided by a drilling having a cross-sectional area which is less than a cross-sectional area of the first pathway section. a first exit pathway section providing unrestricted fluid communication between the volume within the cambox and a common backleak pathway section; characterised in that a second exit pathway section, providing fluid communication between the gallery and the common backleak pathway section, wherein the second exit pathway section is provided by a drilling having a cross-sectional area which is less than a cross-sectional area of the first pathway section.