An improved high pressure reciprocating plunger pump (2) includes a seal for the plunger (8) comprising a V-shaped elastomeric seal member abutted against a backup member (20). The backup member (20) is made from an acetal polymer, e.g. a Delrin R type material, reinforced with high strength fibers, e.g. Kevlar R fibers. This provides a backup member (20) having sufficient resistance to seal member extrusion as well as high strength for resisting side loads imposed by the plunger (8).