All I remember about those valves (depending on what brand you have) there's "special" 0 rings in them. They are NOT a std. 0 ring, as I remember the Deutz ones, have a flat side on the 0 ring.
It's been years since I've had one apart, but I remember having to get the 0 rings from my Deutz dealer, as they are spl...
I think you can still buy them from Peter, at 231 924 3420
SR