Stop key for the valve plunger of a braking assistance servomotor and a servomotor equipped with such a key

A braking assistance servomotor, incorporating a piston hub, a plunger of a distribution valve coupled to an input rod of the servomotor, and a stop key for the plunger, the key comprising a central portion with an end shaped essentially in the shape of a U intended to be engaged in a radial opening in the hub in cooperative engagement with the plunger and, each formed in the shape of a single piece with said central portion, a pair of arms essentially in the shape of an ark of a circle cooperatively engaged with the outer periphery of the hub, said key being made of metal wire, the central portion comprising two substantially parallel lateral mounting strand parts whose first ends are joined together by a central strand part, and each mounting strand part having a second end extended by an outer curved strand part forming the corresponding arm, said radial opening comprising a central portion having axially spaced first and second axial walls, a pair of parallel facing grooves formed in lateral walls of said central opening contiguous with said first axial wall, the axial length of said central portion being greater than the axial length of the grooves, and a projecting portion extending axially from said second axial wall adjacent to said first axial wall.