If you cannot find it, buy the original one which sits on top of the pump, here exampe from the 735
http://www.realoem.com/bmw/showparts...16&fg=05&hl=50
But it mist be available as kit, see here
http://www.bimmerboard.com/forums/posts/217184/
BMW has a kit available that is an external check valve, hose and some clamps. BMW p/n 16 14 9 068 988. Price was less than $3.
http://www.bimmernut.com/%7Ebillr/im...checkvalve.jpg
Thanks to Bill R for the pic.