A vehicle includes variable-ratio gearing driven via a hydrodynamic torque converter. The converter has a hydraulically-engaged lock-up clutch which is temporarily disengaged whenever a ratio is disengaged. The vehicle has a system of hydraulically-engaged clutches and brakes for changing ratio, and a system of hydraulically-engaged brakes for bringing the vehicle to rest. The respective operating pressures in the clutch-and-brake system and the braking system are independently modulated, in order more smoothly to change ratio or bring the vehicle to rest, by enabling a relief valve in each system to open against a determined spring force when pilot pressure exerted on each valve in the closing direction and derived from the operating cylinder of the lock-up clutch is temporarily cut off.