chamber and has both its inlet and its drive chamber connected to a substantially constant pressure charge circuit displacement of the pump is controlled by controlling the fluid flowing from the charge circuit to the inlet the pump outlet is connected to the drive chamber through a combined safety and inching valve which normally blocks fluid flow therebetween but which is responsive to excess pressure at the pump outlet to open and increase the pressure in the drive chamber the valve is also manually operable through a pedal on the vehicle which can be used to destroke the pump in emergency situations or to provide inching control application_number US 61023375 A description referring now to the drawing a hydrostatic transmission constructed in accordance with the principles of the present invention is for illustration purposes only indicated as part of an agricultural tractor 10 which includes a main frame 12 a pair of front steerable wheels 14 a pair of rear traction wheels 16 and an engine 18 the transmission itself includes one or more drive motors 20 drivingly coupled to the rear traction wheels 16 of the tractor 10 a charge pump 22 and a main pump indicated generally at 24 the pumps 22 and 24 are indicated collectively at 26 in fig1 and are illustrated as being coupled directly to the engine 18 the inlet port of the charge pump 22 is connected to.