is delivered through the pipe 32 and the duct 30 30 39 into the cylinders 28 25 and 25 39 of the casing 21 the pistons 29 26 and 26 39 are displaced axially in the direction of the arrow f and through the medium of the thrust ring 38 control the pivoting of the clutch release levers 17a 17b in the direction effecting the release of the friction discs 11a 11b it will have been noted that the pistons 29 26 and 26 39 are adapted to apply a thrust the resultant of which is close to the axis of the sleeve 22 as will have been understood the unit 20 is placed in position simply by transverse engagement of its casing 21 on the sleeve 22 this engagement is possible even when the flexible supply pipe 32 is already connected to this casing the circuit having been previously bled this engagement is facilitated by the gripping handle 36 in practice the unit 20 according to the invention can be held on the sleeve 22 by its own action simply through gravity however it may be provided with various retaining means such as a spring connected to any fixed element and attached to this element or engaging or abutting lugs 37 37 39 which have been referred to above and which surround a projection 41 on the fixed casing in the example illustrated in fig3 and 4 the sleeve 22 is mounted for sliding on the fixed sleeve or 34 trumpet 45 which surrounds the.