I hope you are not being ignored, rather that you have asked a question no-one has the answer for.
This valve is the only one that I have not had apart on my DT969. Here is the exploded view from the parts manual . . . .
. . . . . and the listing for the kit.
The 'Ball' is item G
The 'Cup; is item U
The 'Screw' is item B
The 'Spring' is item F
The 'Spring' is item AB
The 'Valve' is item X
Hmmm, the kit does not seem to have any rubber parts except maybe the Cup. Strange.
It is not much, but maybe a European dealer may have something to help.
Please let us know what you end up doing.
1942 Script GPW (Daily driver).
2 x Diamond T 969.
Kenworth M1A1 Heavy Wrecker.
3ton GS (Blitz) Trailer.
150gal water tanker trailer.