The invention relates to a process and a device for determining the instantaneous value of the amplification provided by a pneumatic-assistance servomotor (14) in a braking device for a motor vehicle, wherein instantaneous values of the relative vacuum level inside the negative-pressure chamber (24) of the servomotor and of the hydraulic pressure at the exit from the master cylinder (16) are measured using sensors (28 and 30), and wherein output signals provided by said sensors are processed by data-processing means (32) so as to compute a pressure difference (ΔP), proportional to the instantaneous amplification provided by the servomotor and, if required, an instantaneous value of the pressure (Prc) inside the working chamber (26) of the servomotor.