Abstract: A mono-seal pump piston comprising a mono-seal assembly including a central seal-supporting hub having a plurality of annular support ribs and a pair of resilient seals oppositely disposed on either side of the supporting hub, the seals including an outer annular sealing lip for sealing engagement with an inner wall of a pump cylinder and an inner annular seaing lip for sealing engagement about the piston rod which passes therethrough. The pump piston is retained on the piston rod by a pair of mounting followers positioned on either side of the piston assembly and held thereon by a nut.