Abstract: The invention provides a pump comprising at least one piston (26, 28) movable in a reciprocating manner within a chamber (12), sealing means (32a, 32b) located between the piston (26, 28) and the interior wall of the chamber (12), and venting means (36) for venting a portion of the chamber (12) bounded by the interior wall, the piston (26, 28) and the sealing means (32a, 32b), wherein the venting means (36) incorporates pressure regulating means (40) such that the pressure to which the said portion of the chamber (12) is vented can be regulated. This reduces the pressure differences across the or each sealing means and thus reduces deterioration and damage.