A hydrostatic propulsion system includes a hydrostatic circuit fluidly connecting left front and right front motors in parallel with an outlet of a pump, left rear and right rear motors in parallel with an inlet of the pump, left front motor and right rear motor in series, and the right front motor and left rear motor in series. A valve system has a first relief valve line providing flow from a right rear motor inlet side to a left rear motor inlet side, and a second relief valve line providing flow from a left rear motor inlet side to a right rear motor inlet side. A first check valve line provides flow from a right rear motor outlet side to the right rear motor inlet side, and a second check valve line provides flow from a left rear motor outlet side to the left rear motor inlet side.