Abstract: A parking brake for automatic transmissions increases the safety of such brakes by arranging the cam, control rod, pawl and lost motion spring so that whenever the shift lever is not moved to its full park position with the engine running, the lost motion spring exerts a force on the mechanism blocking return thereof to reverse position while urging the mechanism in the direction of its park position, thereby preventing the vehicle thought to have been parked from suddenly moving in reverse possibly injuring anyone standing behind the vehicle.