hand be connected to the rear side of the annular flange 48 a ring seal 82 which is accommodated in the transmission piston 26 and comprises two axially spaced circulating sealing lips 84 controls the connection to the interior space of the displaceable valve member 38 and thus to the rear side of the annular flange 48 as long as the radial bore 80 is located axially between the two sealing lips 84 the connection between the radial bore 80 and the interior space of the displaceable valve member 38 is interrupted only after the transmission piston 26 has been moved back relative to the valve member 30 to an extent such that the sealing lip 84 on the left hand side in the figures has moved the radial bore 80 over opposite to the actuating direction of the brake booster is there a fluid connection between the interior space of the displaceable valve member 38 and the annular cavity b so that the above mentioned pressure equalisation can take place once the pressure equalisation has been carried out the force in the actuating direction resulting from the pressure difference at the annular flange 48 ceases so that the compression spring 64 moves the second valve sealing member 52 into contact with the third valve seat 53 and the compression spring 66 presses the displaceable valve member 38 into contact with the annular collar 68 of the force delivery ram 34 the brake assistant function is thus disconnected and the valve seats 50 and 50 u2032 open again if the input member 24 moves back very quickly the brake assistant function can also be disconnected in a second way for a rapid return movement of the input member 24 causes the locking bar.