allow a certain amount of fluid flow once the machine is operating this then unwanted cross port leakage reduces the overall efficiency of the hydrostatic transmission since the effective capacity of flow delivery of pump 12 is decreased cross port leakage also generates substantial heat which has the negative effect of increasing the operating temperature of closed loop circuit 10 this excessive operating temperature is not only a safety issue for machine operators but also reduces the service life of the machine adding an additional oil cooling device not only increases the cost of the machine but also adds complexity and may encumber possible space and location limitations it is thus desirable that an orifice performs its u201c cross port bypassing u201d function only near the neutral or dead band position of the hydrostatic transmission and that thereafter the orifice be disabled during continuous operation of the machine away from the neutral or dead band position fig3 shows a schematic of a special valve block 37 having a directional valve 38 with an orifice 35 movement of directional valve 38 away from its neutral or centered position disables orifice 35 this orifice disablement occurs during normal operation of the hydrostatic transmission and significantly increases the efficiency of the transmission and substantially reduces the working or operating temperature of closed loop circuit 10 fig4 shows a schematic diagram of a hydrostatic transmission closed.