Apparatus for securing and selectively releasing a piston pump from a drive assembly including two collar segments, with one segment fixed to the drive assembly and the other segment movable only linearly transversely with respect to a longitudinal axis of the piston pump, with an over-center mechanism to lock the movable segment via a pair of links. The piston is engageable and disengageable with a drive mechanism in the drive assembly via a pair of interengaging surfaces, with one C-shaped member receiving the other member which is T-shaped.