A hydrostatic transmission, especially for a steer drive of a propulsion and steer drive transmission, having a variable displacement pump, a fixed displacement motor, and a pump displacement control, and including a low pressure fluid source supplying the charging and lubrication systems of the hydrostatic transmission and a high pressure source. The high pressure source, in a normal range of low hydrostatic power pressure values, provides control pressure regulated in proportion to high hydrostatic power pressure to meet the pressure requirements of the pump displacement control which increase with high hydrostatic power pressure. In an abnormal and lower range of low hydrostatic pressure values, the control pressure is regulated to decrease to zero as the low hydrostatic power pressure decreases to a minimum operating pressure value. In the abnormal range, the reduction of control pressure reduces load capacity and thus steer capacity, with decreasing low pressure supplied to the charging and lubrication systems to advise the operator of the abnormal pressures and to discontinue hydrostatic transmission operation to prevent operation with insufficient charging and lubrication pressures.