the tandem brake booster 10 shown in fig1 has a housing 12 made of a front shell 14 and a rear shell 16 joined together by a twist lock connection 18 . the front shell 14 has an integral end plate 15 with a conical section which is substantially parallel to integral end plate 17 on rear shell 16 . the interior of the housing 12 is divided into a first chamber 20 whose limits are defined by the housing 12 and a first wall 24 , a second chamber 26 whose limits are defined by the first wall 24 , housing 12 and a second wall 28 , and a third chamber 30 whose limits are defined by the second wall 28 , housing 12 and a cylindrical projection 32 extending from the rear shell 16 . the real shell 16 is fixed to a stationary member 33 of a vehicle by a plurality of bolts 35 . the first wall 24 includes a diaphragm 22 , a backing plate 21 and a hub member 34 . a first bead 23 on diaphragm 22 is retained by band of the twist lock connection 18 and a second bead 25 which is located in groove 27 on hub member 34 holds backing plate 21 against a shoulder 29 . hub member 34 has a cylindrical projection 38 that extends through an opening 40 in end plate 15 . a push rod 41 connects projection 38 with a master cylinder , not shown . hub member 34 has a central flange 42 with a plurality of slots 44 , 44 &# 39 ;. . . 44 &# 34 ; located therein for receiving ends 46 , 46 &# 39 ;. . . 46 &# 34 ; of lever arrangement 48 which connects the second wall 28 with the first wall 24 . the second wall 28 includes a disc member 50 , a diaphragm 52 , and seal member 54 . diaphragm 52 has a first bead 56 that snaps over the end of disc member 50 and a second bead 58 is held against bead 23 by band of the twist lock connection 18 . fastener members 60 , only one of which is shown , are attached to the disc member 50 . arms 62 of the lever arrangement 48 has one end 64 pivotally attached to fastener member 60 by pins 66 . projection 32 that extends from end wall 17 of shell 16 has a series of slots 68 , only one of which is shown , with a lip 70 located at the bottom of each slot 68 . arms 62 engage lips 70 such that a fulcrum is produced in order to transfer force between the second wall 28 and central flange 42 . a control mechanism 72 has a cylindrical body 74 with a seal 76 that engages projection 32 and an annular surface 78 that engages arms 62 between end 46 , 46 &# 39 ;. . . 46 &# 34 ; and fulcrum lips 70 . a bore 80 in the cylindrical body 74 has passages 82 and 84 that connect chamber 30 which chamber 26 . a concentric cylindrical member 86 has a seal 88 that engages bore 80 and an opening 85 through which passage 84 is connected with an interior bore 90 . bore 90 is connected to passage 82 by passage 92 . a reaction disc 94 is located between the bottom of bore 80 and the end of cylindrical member 86 . a plunger 96 located in bore 90 has a series of slots 98 that provide for communication between passage 92 and vacuum seat 100 . plunger 96 has an atmospheric seat 102 . a poppet valve 104 located in bore 90 has a first end 106 secured to cylindrical member 86 and a second end 108 that is free to move within bore 90 . a first spring 110 acts on push rod 112 attached to plunger 96 to urge atmospheric seal 102 into engagement with free end 108 . a second spring 114 located on piston rod 112 acts on the free end 108 to assist in establishing an atmospheric seal between plunger 96 and poppet valve 104 . in an internal combustion engine , vacuum is produced at the intake manifold and as a result such vacuum is used in conjunction with air at atmospheric pressure as a motive force for operating many components of a vehicle . in a vehicle equipped with a tandem brake booster 10 shown in the drawing , vacuum is communciated through check valve 120 into chamber 20 . vacuum in chamber 20 evacuates air from chamber 26 by way of passage 82 , bore 90 , passage 92 , passage 84 , chamber 30 and conduit 122 . when an operator desires to effect a brake application , an input force is applied to push rod 112 to move plunger 96 toward disc 94 . initial movement of push rod 112 activates poppet valve 104 by allowing spring 114 to move free end 108 into engagement with vacuum seat 100 to interrupt vacuum communication to chamber 26 . further movement of push rod 112 moves atmospheric seat 102 on plunger 96 away from free end 108 to allow air to enter chamber 26 by way of bore 90 , slots 98 , passage 92 , bore 80 , and passage 82 . with air in chamber 26 and vacuum in chambers 20 and 30 , a pressure differential is created across walls 24 and 28 . this pressure differential acts on wall 24 to produce a first output force that is directly communicated to push rod 41 by way of hub member 34 . movement of wall 28 which is in the opposite direction of movement of wall 24 causes arms 62 to pivot about fulcrum lip 70 and provide hub member 34 with an additional or additive force to produce the operational force for push rod 41 . the input force applied to push rod 112 to move plunger 96 acts through reaction disc 94 to move cylndrical body 74 toward hub member 34 as wall 24 moves toward chamber 20 . the resistance to movement of push rod 41 produces a reaction force that is carried back through annular surface 78 and into disc 94 to balance the input force applied to push rod 112 . since the annular surface 78 engages arm at a position between fulcrum 70 and end 46 , the movement of control member 72 as compared with the total movement of push rod 41 is equal to the ratio of the distance between end 46 and fulcrum 70 and between the annular surface 78 and fulcrum 70 . depending on the desired operational force , with end plate 17 fixed to stationary support , an axial deflection force is produced in the tandem brake booster 10 . the conical - shaped end members 15 and 17 tend to resist such axial deflection . in addition , when a maximum operational force is desired , the conical - shaped walls 24 and 28 nestle into the end member 15 and 17 to provide for a most efficient use of space . on termination of the input force to push rod 112 , spring 110 initially moves atmospheric seat 102 into engagement with free end 108 to interrupt communication of air to chamber 26 . further movement of plunger 96 by spring 110 moves free end 108 away from vacuum seat 100 . with vacuum seat 100 opened , vacuum in chambers 30 and 20 evacuate air from chamber 26 . as the pressure differential across walls 24 and 28 equalizes , spring 140 acts on hub member 34 to move wall 24 toward end plate 17 . at the same time , a force acting on end 46 of lever 62 moves wall 28 toward end plate 15 . when the fluid pressure in chambers 20 , 26 , and 30 equalizes , cylindrical member 74 engages end plate 17 to limit the movement of walls 24 and 28 toward each other to produce a rest position as shown in the drawing . in the absence of vacuum in chambers 20 and 30 , an operator must manually provide an input to push rod 41 . the input applied to push rod 112 moves plunger 96 into engagement with disc 94 to provide cylindrical member 74 with linear movement . annular surface 78 acts on lever arm 62 to transmit the linear movement to hub member 34 . as hub member 34 moves toward end plate 15 , lever arms 62 pivot about pin 66 since wall 28 remains stationary . slots 68 in projection 32 are such that when hub member 34 reaches its maximum stroke , lever arms 62 are still retained in slots 68 . on termination of the manual input , return spring 140 returns the wall 24 to the position illustrated in the drawing .