to explain the invention , exemplary reference is made to a pneumatic pump , in which case the designs described also apply to pumps for fluids . the pump 1 shown in the illustrations is intended to be installed in a vehicle seat and serves for filling seat bladders to change the contour of a vehicle seat in the sitting or backrest area , for example . the pump 1 comprises a housing 2 ( especially made of plastic ) on which an outlet connection piece 3 has been form - fitted . the side of the housing 2 supporting the outlet connection piece 3 will also be referred to as upper side 4 below . a motor 6 , especially an electric motor , has been flanged onto the lower side 5 of the housing 2 . the housing 2 comprises an upper part 7 and a lower part 8 , in which case the upper part 7 supports the connection piece 3 and forms the upper side 4 . two plate - shaped supports — namely an upper support 9 and a lower support 10 — are clamped sandwich - like between the upper part 7 and the lower part 8 . between the upper support 9 and the upper part of the housing 7 , a plate - shaped valve membrane 13 made of an elastic material ( e . g . a rubbery polymer ) has been clamped in . four pot - shaped depressions that create pump chambers 15 have been form - fitted in the pump membrane . in its upper border , the pump membrane 14 has a circumferential bulge 16 clamped between the upper support 9 and the lower support 10 . on the upper part of the housing , peripheral arms 18 extending downwards have been form - fitted along the middle longitudinal axis 17 of the pump . on the free end of two diametrically opposing arms 18 , a rear grip element 18 has been fixed or form - fitted that , by creating an axially effective positive - locking fit , grips an opposing element 20 on the lower part of the housing 8 from behind . as a result of this , the valve membrane 13 , the upper support 9 , the pump membrane 14 and the lower support 10 are axially tensed between the upper part of the housing 7 and the lower part of the housing 8 . in the lower part of the housing 9 , there is a turnstile 23 obliquely positioned with respect to the middle longitudinal axis 17 that has been mounted on an eccentric 25 with an axis 24 protruding from its underside . the eccentric 25 , in turn , is connected in a fixed rotary way to a drive shaft 26 of the motor 6 . the pump membrane 14 has on the underside , in the area of the pump chambers 15 , a tappet - shaped extension 27 with a head - shaped end 28 fixed in a recess 29 of the turnstile . each pump chamber 15 is assigned to an inlet valve 30 and an outlet valve 33 . the outlet valves 33 are arranged in an outlet port 34 that extends inwards from the pump chamber 15 all the way to the inlet connection piece 3 and finally ends in the surroundings ( i . e ., the environment ). the inlet valves 30 are arranged in an inlet port 35 in which — with respect to the inflow direction 36 — suctioned air flows towards the pump chamber , the end 37 located upstream ends in a housing space 38 , in which a drive unit 39 for actuating the pump chambers 15 ( i . e . for enlarging and reducing them ), in this case the turnstile 23 and the eccentric 25 , are arranged . the inlet valves 30 and the outlet valves 33 have in each case a valve element 30 a that is freely cut as tongue - shaped parts from a valve membrane 13 forming an entire membrane and that in each case controls an inlet opening 30 and an outlet opening 33 c located in the upper support 9 . each one of the valve elements 30 a acts with a seal seat 30 b protruding from the underside of the upper part of the housing 7 ( fig3 & amp ; 6 ). the seal seats 30 b are sections of a ring - shaped closed seal seat 32 that separates the lower pressure side of the pump 1 from its pressurized side . the valve seats 33 b assigned to the outlet valves 33 protrude from the upper side of the upper support 9 . in the situation shown in fig7 , the valve element 33 a lies on top of the valve seat 33 b that encompasses the outlet opening 33 c . on the upper side of the valve membrane 13 , all inlet ports 35 end in a collecting space 41 ( located above the valve membrane 13 ) arranged centrally on the pump in the direction of the arrow 40 in fig1 in the top view . the collecting space 45 is circumscribed towards the top by the upper part of the housing 7 and towards the bottom by a central area of the valve membrane 13 that comprises the valve elements 30 a of the inlet valves 30 . apart from the inlet and outlet vales 30 , 33 , the pump 1 is equipped with a pressure relief valve 43 developed between a central area of the valve membrane 13 that forms a valve element 43 a and a seal seat 43 b of the upper support 9 . the valve element 43 b is centrally arranged in the collecting space 41 and interfused by a central connection opening 44 . thus , the valve element 43 a is the peripheral area of the valve membrane 13 that circumscribes the connection opening . the valve element 43 a lies on a ring - shaped seal seat 43 b of the upper support 9 that circumscribes the pressure relief opening 43 c . to increase the sealing effect , the peripheral area mentioned above has a sealing lip 45 on its underside . an opening 42 a interfused with the pump membrane 14 is located centrally in the latter . below the opening 45 , the central area of the pump membrane 14 surrounded by the pump chambers 15 is supported by an area 42 of the lower support 8 that bulges out upwards , in which case it is also interfused with an opening 46 . thus , the section of the inlet port 35 extending away from the valve element 30 a of the inlet valves 30 against the inflow direction 36 is created by the collecting space , the connection opening 44 and subsequent openings 44 , 43 c , 42 a and 46 , in which case the inlet port 35 ends with the opening 46 in the housing space 38 , which is circumscribed by the lower support 10 and the lower part of the housing 8 . a largely cylindrical connection piece 47 open towards the upper side of the housing 4 juts out from a central position of the upper part of the housing 7 . the interior of the connection piece creates a spring space 49 for receiving a spring element 48 , namely a threaded compression spring . on its front side facing away from the upper part of the housing 7 , the connection piece 47 has an assembly opening 50 over which the spring element 48 can be inserted into the spring space 49 . the spring element 49 supports itself with its lower end on the valve element 43 a of the pressure relief valve 43 by means of an intermediate layer of a supporting ring 52 , thus impinging on the former in closing direction 31 . the other end of the spring element 48 , on the other hand , supports itself on a stop element 51 arranged on a fixed axis ( with respect to the medium longitudinal axis 17 , inside an adjustment area 53 formed by a longitudinal section of the inner wall 54 of the spring space 49 ). the axially fixed fixation of the stop element 51 is accomplished by making its dimension transversal to the medium longitudinal axis 17 slightly larger than the inside diameter of the adjustment area 53 . in the examples shown in the drawings , the inner wall 54 of the adjustment area is cylindrical or extends on the outer surface of a cylinder and has an inside width or diameter 55 that is smaller than the dimension of the stop element 51 in a direction running transversally to the medium longitudinal axis 17 . the stop element 51 is preferably a metallic sphere with a diameter 56 slightly larger than the inner diameter 55 of the adjustment area 53 . owing to the dimensions mentioned above and the connection piece &# 39 ; s softer plastic material compared to the metallic sphere , the stop element 51 digs itself into the contact area 57 by forming a ring - shaped cavity 58 in the inner wall 54 of the connection piece 47 . the dimension relationships mentioned above have been chosen in this case so that the clamping force with which the stop element 51 is held in the adjustment area 53 is larger than the force exerted by the spring element 48 in axial direction . to set a certain limit pressure ( i . e . a pump chamber pressure ) to which the pressure relief valve 43 should react , the stop element 51 is inserted into the spring space 49 through the assembly opening 50 while the pump is being assembled and moved in closing direction 31 until the spring force exerted by the threaded compression spring 48 correlates with the desired limit pressure . in the design variant shown in fig9 & amp ; 10 , the spring space 49 ′ is not connected to an inlet port . here , the pressure between the spring space and the surroundings is compensated through a connection opening 59 interfused with the wall of the connection piece 47 . as in the design variant described above , the valve element 43 a lies on a ring - shaped seal seat 43 b ′, though it does not border an opening but merely a recess 60 dosed towards the bottom . the inlet port ( not shown ) is therefore not connected to the spring space 49 . the way the pump works will now be explained : when the air from the surroundings or from the housing space 38 is suctioned , it enlarges the pump chambers 15 by moving the extensions 27 of the pump membrane 14 downwards . the air [ flows ] through the openings 46 , 42 a , 43 c and 44 into the collecting space 41 ( arrows 63 ) and from there through the inlet valves 30 to the pump chambers 15 ( fig6 ). when this occurs , the valve element 30 a is elastically deformed and moved downwards ( arrow 64 ) and as this happens , it lifts off the seal seat 30 b . the valve elements 30 a and the valve elements 33 a of the outlet valves 33 are not as thick as the remaining valve membrane 13 , so that in each case a free space is created above and below the valve elements that allows the valve elements to be lifted from the respective valve seat . if the extensions 27 move upwards , the pump chambers 15 are made smaller and air is driven out of them . the air that was driven out flows through the outlet openings 33 c in accordance with arrow 66 in fig7 and reaches a pressure space ( see also fig3 ) bordered by the valve membrane 13 , the upper part of the housing 7 and the seal seat 32 . the pressure space is connected to the outlet port 34 of the outlet connection piece 3 . if excess pressure exceeds the limit pressure , the pressure relief valve 43 reacts . when this occurs , the valve element 43 a is lifted off the valve seat 43 b , 43 b ′, so air can reach the spring space through the connection opening 44 available in the valve element 43 a ( see arrow 70 in fig8 ). in the design variant shown in fig9 and 10 , pressure is compensated with the surroundings through the connection opening 59 , otherwise via the inlet port 35 . the impingement of the valve element 43 a with excess pressure takes place through another pressure space 68 arranged below the valve membrane and bordered by it and the upper support 9 . the pressure space 67 mentioned above arranged above the valve membrane is connected to the pressure space 68 arranged below the valve membrane through openings 69 .