a brake power booster is shown in fig1 and 2 and includes a vacuum housing 1 which is sub - divided by an axially movable wall 2 into a working chamber 3 and a vacuum chamber 4 . the axially movable wall 2 includes a diaphragm plate 5 made from deep - drawn sheet metal and a flexible diaphragm 6 abutting thereon . the diaphragm forms a rolling diaphragm and defines a seal between the outer circumference of the diaphragm plate 5 and the vacuum housing 1 , only a portion of which is shown in the drawings . a control valve 9 actuatable by a piston rod 8 is provided with a control valve piston 10 connected to the piston rod 8 , which valve piston releases valve openings in a control valve housing 11 in manner such that the working chamber 3 is , in the depicted inoperative position of fig1 connected to the vacuum chamber 4 . connection of the working chamber to the vacuum chamber is through the intermediary of air feed channels 12 running laterally inside the control valve housing 11 which discharge onto the face of a poppet valve 23 at the circumference of the control valve housing 11 . on actuating the control valve 9 , that is , when the piston rod 8 is displaced axially , communication between the vacuum chamber 4 and the working chamber 3 is interrupted and the working chamber 3 is connected to the atmosphere so that the movable wall is caused to move toward the vacuum chamber 4 . the brake power is transmitted by way of a rubber - elastic reaction disc 13 seated in an indentation on the face of the control valve housing 11 and a push rod 14 provided with a flanged head 15 onto an actuating piston of a master cylinder ( which is not depicted ) of the brake system . the master cylinder is arranged at the vacuum side end of the brake power booster . a pressure spring ( not depicted in the drawing ) bearing against the front wall of the vacuum housing 1 on the vacuum side of the booster and on a flange , maintains the movable wall 2 in the depicted initial at rest brake released position . on actuation of the braking pedal , the piston rod 8 and the valve piston 10 are moved to the left , in opposition to the force of a spring 24 . the spring 24 of the poppet valve 23 thereby urges the valve to the left onto the seat 22 in the control valve housing 11 , and the air feed channel 12 is closed . in the course of further movement , the seat 18 at the valve piston 10 moves away from the poppet valve 23 , and the outside air passage 26 is opened . at that point atmospheric air may flow through the outside air passage 26 in the control valve housing 11 toward the right hand side of the valve piston 10 and the vacuum in the working chamber 3 is thereby reduced . the force generated due to the pressure differential existing on the right and on the left hand side of the movable wall 2 displaces the wall to the left in opposition to the force of the pressure spring , and displaces the push rod 14 and the piston in the master cylinder . the pressure which is built up in the master cylinder exerts a reaction force on the reaction disc 13 by way of the piston and the push rod 14 . the reaction pressure is also transmitted onto the valve piston 10 , the latter thus being displayed to the right , whereupon the seat 18 sealing engages with the poppet valve 23 . in doing so , the air feed channel 12 and the outside air passage 26 are closed . the valve piston 10 has now assumed its so - called &# 34 ; alert &# 34 ; position . in the maximum braking position , the outside air passage is constantly open so that the maximum pressure differential , and thus the maximum boosting pressure , is available at the movable wall 2 . the runout pressure of the booster has thus been reached . a further increase in the master cylinder pressure can only be achieved by an additional force applied on the piston rod 8 . on releasing the braking pedal completely , the valve piston 10 returns to its initial position , the outside air passage 26 is closed and the vacuum passages 12 , 27 , 28 are opened . due to the vacuum balance at the movable wall 2 resulting thereform , a force is no longer exerted on the wall and the force of the spring arranged in the brake booster housing 1 is sufficient to urge the movable wall 2 back to its inoperative position . the piston of the master cylinder then also returns to its inoperative position . if a blocked pressure is built up in the vacuum chamber 4 , such as , for example , when the brake booster is operated without a vacuum or with vacuum passages 12 , 27 , 28 closed a component of force acting in a radial direction of the brake power booster is being generated . due to the effect of this radial force , the rolling diaphragm 6 , which has its edge bead 38 clamped in a peripheral groove of the control valve housing 11 , can be detached from the groove as a result of which the brake booster will become inoperative . in order to prevent the rolling diaphragm 6 from detaching from or slipping out of the peripheral groove of the control valve housing 11 , a retaining element 35 is provided which is arranged in an annular chamber b defined by the surface of the control valve housing 11 and a neck portion 31 of the rear vacuum housing shell 29 . the retainer secures the edge bead 38 of the rolling diaphragm 6 against the dislodging effect of the radial force . as shown in fig1 and 2 , the control valve housing 11 is provided with an air feed channel 27 extending transversely to the longitudinal axis of the booster , which channel connects the valve chamber 28 behind the control valve 9 with the working chamber 3 and through which channel outside air flows on actuation of the braking pedal . the retaining element 35 is axially nondisplaceably held on the control valve housing 11 in this air feed channel 27 . as can be seen in fig2 the retaining element 35 can preferably serve , at the same time , as a support for an air filter and / or damping element 36 , whose function is principally to reduce noises occurring during the rapid inflow of outside air into the working chamber 3 due to high flow rates . the damping element 36 can advantageously be formed by means of a polyurethane foam ring which has a damping effect on the outside air flowing at a high rate into the working chamber 3 so that the air is forced to spread out in front of the polyurethane foam ring and to flow through the latter at low rate . the retaining element 35 shown in fig3 includes a first ring 41 as well as a second ring 42 , both rings 41 , 42 being co - axially arranged at an axial distance from each other and being connected with each other by means of a plurality of webs 43 extending axially between the two rings 41 , 42 . slits 44 are formed between the individual webs 43 and permit bleeding of the brake booster , which bleeding is required for proper functioning of the booster . fig4 shows the retaining element 35 according to invention installed in the vacuum housing 1 of the vacuum - operated brake power booster . this retaining ring includes two opposite projections 45 being shaped in the form of a segment of a circle and being provided on the radially inner surface of the second ring 42 . when the retaining element 35 is installed , the projections 45 snap into the air feed channels 27 extending radially in the control valve housing 11 and bear against the wall of the air feed channels 27 . the arrangement illustrated in the drawing shows a slight distance between the surface of the edge bead 38 and the radially inner surface 46 of the first ring 41 , however it , is of course , also possible to dimension the first ring 41 so as to abut directly with its radially inner surface 46 on the edge bead 38 of the rolling diaphragm 6 .