Abstract: There is disclosed an automotive hydraulic brake booster in which a driver feels a brake pedal moving smoothly as he or she pushes down on the brake pedal. The booster includes a housing connected with a master cylinder, an input rod, a booster chamber formed in the housing, a power piston sliding inside the booster chamber, and a reaction piston connected to the input rod and inserted in a hole formed in the power piston. A linkage is connected to a valve for boosting the force supplied to the master cylinder and connected to the reaction piston. The reaction piston can move a given distance relative to the power piston in response to the input rod, this distance being determined by a stop. The stop and the connection between the input rod and the reaction piston are on the same side of the connection between the linkage and the reaction piston. A reaction chamber is formed in said hole by the reaction piston.