and a seal to the diaphragm 38 around the diaphragm opening 43 this is best shown in fig2 and 9 the check stem 34 extends through an axially extending aperture 35 in the check plate 32 the check spring 36 is compressed by flow pressure in the system against the upstream side of the upstream check disc 42 during normal operation and allows water flow through the diaphragm opening 43 of the diaphragm 38 the check plate assembly 30 is enclosed within a void defined by the cap 22 and a centrally positioned valve housing 60 the valve housing 60 includes an axially extending housing wall 62 a vent 64 communicates through a port 65 with the interior of the valve housing 60 as shown in fig2 5 and 10 a downstream check plate assembly 80 is positioned on the downstream side of the backflow housing 60 and includes a stem 82 integrally formed with a check disc 84 with a spring 86 carried on the stem 82 the check disc 84 is backed by a washer 88 the downstream side of the valve housing 60 includes three wedge shaped flow ports 90 defined by three spokes 92 that permits water flow downstream through the flow ports 90 and downstream out of the backflow preventer 10 the spokes 92 support a concentric hub 94 in which is formed an aperture 66 in which the stem 82 of the check disc 84 is mounted for axial movement normal flow through the backflow preventer 10 compresses spring 86 as the check disc 84 is urged downstream and allows flow through the ports 90 a tailpiece 98 is captured in a threaded union nut 100 which is threaded onto the downstream end of the housing 60 a rubber washer 102 seals.