A hydraulic system includes a charge pump which supplies fluid to variable displacement main pump, a fluid reservoir and valve unit. The valve unit includes a hollow housing having a first end communicated with the reservoir and a second end communicated with the charge pump and the main pump. A housing valve land member is located between the first and second ends. A valve member is movably mounted in the housing. The valve member has a valve disk attached to first and second valve stems which project from opposite sides of the valve disk. The valve disk has a central position wherein the valve disk is received by the ring. The valve stems are slidably supported by stem support members at opposite ends of the housing. A hollow sleeve projects from an end of the valve unit. A spring is mounted in the sleeve and is engagable with an end of one of the valve stems and is biased to urge the valve member away from the sleeve.