Abstract: A pneumatic brake booster for use in a motor vehicle having a feeler (42) mounted to slide on a plunger (32) located at the end of a control rod (22) and pivoting key (62) for axially blocking the movement of the feeler (42) with respect to the plunger (32) during a normal brake application and unblocking the movement during an emergency brake application to reduce the length of a plunger-feeler assembly and thus increase the jump phase of an output force.