A tractor braking system (10) having independent brake circuits (L,R) for each of a left and a right wheel is provided. Each circuit is activated by a respective brake pedal (16,17). The system comprises a releasable locking mechanism (25) to lock the two pedals together and sensors (50) to detect depression of the respective pedals (16,17). In response to a detection of only one pedal being depressed, warning means are provided to indicate to a driver that the pedals are in an unlocked state. A sensing of simultaneous depression of the pedals may be used to indicate that the pedals are in a locked state.