A variable displacement radial ball pump (11) is disclosed of the type in which the balls (59) are restrained by a cam ring (25), which is pivotable about a cam pivot pin (27) to vary the displacement of the pump. In accordance with the invention, in order to return the pump to absolute zero displacement, the pump is provided with a detent mechanism (65;83). The cam ring 25 has a neutral region (73) and a ramp surface (77), and a detent member (69;89) is biased into engagement with the neutral region when the cam ring is in neutral (FIGS. 2 and 3). When the cam ring is displaced from neutral, the detent member engages the ramp surface, against the bias force of a spring (71;91), which exerts a force tending to return the cam ring to neutral in the absence of an input to the control assembly (33).