A pump for a pressure washer. The pump includes a cam housing, a piston assembly disposed within the cam housing, a rotary shaft assembly disposed within the cam housing, a journal plate attached to the cam housing, a head assembly attached to journal plate and valve assembly operatively connected to the head assembly. The rotary shaft assembly includes a cam shaft, a vertically-extending oil tube partially disposed within a central portion of the cam shaft, a cam threaded onto the cam shaft and a thrust washer positioned adjacent the threaded cam. The piston assembly includes a plurality of pistons, preferably three, in mating contact with the thrust washer. Each piston includes a piston spring cap at one end, a spring disposed over the piston and engaging the piston spring cap, and a seal retainer having a low pressure seal at one end and a high pressure seal at the other end. The seal retainer includes an opening between the low pressure seal and the high pressure seal such that fluid passing through the high pressure seal is captured by the low pressure seal for recycling back into the head assembly. The head assembly, in combination with the valve assembly, forms a castellated assembly of a plurality of towers, preferably three, one for each piston. Each tower includes a low pressure valve assembly and a high pressure valve assembly that act as check valves. The low pressure valve assembly allows intake water to flow from the journal plate to a castellated tower channel. The high pressure valve assembly allows water to flow from the channel to the outlet fitting. The pressure washer can be mounted in either the vertical or horizontal orientation.