portion 1066 of the piston 4 in this case the material of the friction means fills the space between the piston 4 and the collar portion 1166 the collar portion 1166 is thus encapsulated in the friction means 60 in another version the friction means 60 can be snap fitted on the projecting portion 1066 the collar portion 1166 of which constitutes a bead thus the projecting portion 1066 is snap fitted in a hole 1266 in the friction means 60 of castellated contour for example as is best seen in fig2 the friction means 60 includes at least one lip 1366 engaged in a groove of a projecting portion 1066 of the piston 4 this groove being delimited by the piston 4 and the bead 1166 in another version the friction means 60 has at least one resilient lug engaged in the hole 161 in the piston 4 and provided with claws at its free end thus in fig2 the ring 60 has a plurality of lugs 1466 with claws extending through the hole 161 and this hole is preferably flared towards the radial plate 15 so that the lugs 1466 are able to be retracted during their forcible engagement in the hole 161 and to be then deployed towards the outside once the claws of the lugs 1466 have come out on the other side these claws come into engagement with the face of the piston that faces towards the wall 3 snap fitting means are thus interposed between the piston 4 and the friction means 60.