A pneumatic servomotor for assisted braking of a motor vehicle, said servomotor having a rigid casing with a transverse partition wall located therein to define in an airtight manner a front chamber connected to receive a first pressure (vacuum) and a rear chamber connected to selectively receive said first pressure and a second pressure (atmospheric pressure), said partition wall having an integral piston with a bore therein for receiving a control rod, said control rod moving within said bore in response to axial input force exerted in the forward direction against a return force applied to the control rod by a return spring towards either an intermediate actuation position or an end actuation position, said intermediate actuation position and an end actuation position being determined by a predetermined speed at which said input force is applied to said control rod;
- a plunger located in said bore adjacent a front end of said control rod, and a three-way valve, including at least an annular seat borne by a rear section of said plunger, said annular seat allowing said rear chamber to be opened to said front chamber and receive said first pressure from said front chamber when said control rod is in a rest position and closed to said front chamber when an input force moves said plunger from said rest position to allow said second pressure to be gradually supplied to said rear chamber as said plunger moves toward said intermediate actuation position and said end actuation position;
- said first pressure in said front chamber and said second pressure in said rear chamber creating a pressure differential that acts on said partition wall to develop a force that moves said movable wall toward the front chamber, a finger having a front face and a rear face, said finger being slidingly fitted at a front end of said plunger and biased by said plunger so that said front face of said finger comes into contact with a reaction disk that is integral with the moving piston when said control rod is in said end actuation position so as to transmit a reaction force from said moving piston to said plunger and to the control rod;
- and a unidirectional clutch device, that includes at least one coaxial tubular sleeve that slides on said plunger, said finger being defined by a front end of said sleeve, and a latching element that is movable between an inactive position and an active position as a function of said end actuation position of the control rod, said latching element when in said active position co-operates with a locking element of sleeve to lock said sleeve in an end front axial position in such a way that said finger may lock the movement of said moving piston, independently of the movement of said plunger and control rod, said sleeve being characterised by a first adjusting part, situated between said front and rear faces of said finger and a second adjusting part, situated between said rear face of said finger and said locking element, said first and second adjusting parts being plastically deformable at least in the axial direction to allow for the adjustment of an intermediate actuation position and said end actuation position of said control rod prior to the assembly of said servomotor, said latching element including a substantially annular key that surrounds said sleeve with a given clearance and is driven by said piston when an input force is applied at a predetermined speed so as to rock about a generally transverse axis in such a way that a peg on said annular key abuts against a rear transverse face of said sleeve to define said locking element, said annular key having an axial sectional view in a shape of a tee, said tee having a vertical branch of that is substantially radially directed and traversed with said sleeve such that a front horizontal half-branch of a face of said tee radially faces said sleeve, protrudes from said peg and is axially directed by being resiliently biased against the rear of one of said opposite walls of the cavity by two compression springs that are located between a front wall of said cavity and blind holes on both sides of the sleeve to center said tee of said key, said tee being without any axial clearance between two opposite walls in a cavity traversing said piston while allowing a rocking motion of said key inside the cavity to lock said sleeve within said piston.