An air compressor has a piston (3) reciprocable to compress air within a cylinder (2), a clutch (16) to transmit drive from a drive input shaft (12) to the piston when compression is required, a piston (27) arranged to be responsive to pressure resulting from the compressor output operable to disengage the clutch (16) and thereby interrupt drive to the compressor piston when the pressure reaches a predetermined value, and a passage (37) to provide a leakage path for the pressure applied to the piston (27) to enable the pressure to decay in a controlled manner when further operation of the piston is required, whereby engagement of the clutch is permitted to occur progressively and shock loadings on the compressor components are minimized.