A method of calibrating a hydraulically operated park brake of a vehicle, comprising steps of: a. propelling the vehicle using the transmission, with all brakes off; b. monitoring a pressure condition in a hydrostatic power unit of the transmission, including recording a base pressure value in the unit; c. controlling the park brake in a predetermined manner using a first control signal value, and determining a resulting change in a pressure condition in the hydrostatic power unit, characterised in that i. if the resulting change in the pressure condition in the hydrostatic power unit is more than a first predetermined value, then releasing the park brake and repeating steps a.- c. to control the park brake using a second control signal value expected to produce a lower resulting change in the pressure condition in the hydrostatic power unit; and ii. if the resulting change in the pressure condition in the hydrostatic power unit is less than a second predetermined value, then releasing the park brake and repeating steps a.- c. to control the park brake using a control signal value expected to produce a higher resulting pressure in the hydrostatic power unit; and d. if the resulting pressure in the hydrostatic power unit after performing steps a.- c. is between the first predetermined value and the second predetermined value, then recording the control signal value as the calibration value. a. propelling the vehicle using the transmission, with all brakes off; b. monitoring a pressure condition in a hydrostatic power unit of the transmission, including recording a base pressure value in the unit; c. controlling the park brake in a predetermined manner using a first control signal value, and determining a resulting change in a pressure condition in the hydrostatic power unit, characterised in that i. if the resulting change in the pressure condition in the hydrostatic power unit is more than a first predetermined value, then releasing the park brake and repeating steps a.- c. to control the park brake using a second control signal value expected to produce a lower resulting change in the pressure condition in the hydrostatic power unit; and ii. if the resulting change in the pressure condition in the hydrostatic power unit is less than a second predetermined value, then releasing the park brake and repeating steps a.- c. to control the park brake using a control signal value expected to produce a higher resulting pressure in the hydrostatic power unit; and i. if the resulting change in the pressure condition in the hydrostatic power unit is more than a first predetermined value, then releasing the park brake and repeating steps a.- c. to control the park brake using a second control signal value expected to produce a lower resulting change in the pressure condition in the hydrostatic power unit; and ii. if the resulting change in the pressure condition in the hydrostatic power unit is less than a second predetermined value, then releasing the park brake and repeating steps a.- c. to control the park brake using a control signal value expected to produce a higher resulting pressure in the hydrostatic power unit; and d. if the resulting pressure in the hydrostatic power unit after performing steps a.- c. is between the first predetermined value and the second predetermined value, then recording the control signal value as the calibration value.