Abstract: A pneumatic servo for assisting braking. The servo has a casing with an axis of symmetry which is divided in leaktight fashion by a movable wall structure into a front chamber and a rear chamber. The front chamber is permanently connected to a source of partial vacuum while the rear chamber is connected selectively to the front chamber and to air in the surrounding environment by a three-way valve. A sleeve divided an annular bore located in a projection that extends through the movable wall through the casing into a static volume and a concentric annular volume. The sleeve engages the projection to position the three-way valve in the bore. A return spring located in the static volume engages the control rod while an end plug engages the sleeve to seal the static volume from the surrounding environment. The annular volume being is respectively connected by first and second openings to the surrounding environment and bore.