Sounds just like a monovalve:
http://www.peachparts.com/shopforum/showthread.php?p=1026761#post1026761
See the links on my entry on the attached thread.
Also look at this:
http://www.peachparts.com/shopforum/showthread.php?p=1023864#post1023864
If it is the monovalve then you need to get a replacement center piece: Bosch part number 1 147 213 007. I got mine at Olympic Auto Parts for ~$60, but you can get it on line here for less if you want to wait for delivery (I didn't). The dealer wants about $100, but some will say you need the whole valve for over $330!