Abstract: A hydrostatic transmission connected by means of a power transmission shaft to a speed reducing transmission, the housing structure surrounding said transmissions defining a first and second internal volumes. The first internal volume providing a receptacle for the hydrostatic fluid and for containing the hydrostatic transmission components. The second internal volume, which may or may not contain the gear train components, operates as an overflow receiver for fluid transferred from the first internal volume. A duct disposed in the housing provides the only flow path between the volumes and any fluid volume change in the first internal volume due to temperature variation is translated in a change in depth in the overflow receiver. Preferably, the duct operates in accordance to the siphon principle and catering for fluid volume change internally is a significant improvement over current practice where reliance is placed on external paraphernalia to achieve this end.