A transmission assembly for a self-propelled machine includes a transmission housing fitted with an input shaft, an output shaft, a mechanism for reversing the rotational drive direction of the output shaft, and a brake mechanism, each mechanism including a control, the direction reversing and brake controls carried by the housing being mounted respectively movable, one between at least three positions: a neutral, a forward and a reverse position, the other between at least a first braking position and a non-braking position, the brake control being couplable to a brake control member that can be actuated by the operator, in order to shift the brake control from the non-braking to the first braking position. The brake control has a separate second braking position, and the transmission assembly includes return elements capable, in the neutral position of the direction reversing control, of returning the brake control to the second braking position.