Hydrostatic vehicle steering system with supplemental charge pump

A supplemental charge for a hydrostatic vehicle steering system is provided by coupling the output of the transmission oil pump (54) to the outlet of the steering charge pump (36) and providing a check valve (62) to prevent oil flow from the transmission oil pump (54) to the steering charge pump (36) unless the charge pump outlet pressure drops below the operating pressure of the transmission pump (54). When the steering charge pressure does drop below the transmission oil pressure, the check valve (62) opens and oil is provided to the main steering pump (40) by the transmission oil pump (54). A pressure transducer (64) in the oil line (60) activates a warning signal in the event the check valve (62) is open more than a few minutes to alert the vehicle operator to a system failure.