[s-cars] P/S pump plug leaking

[Lino M. Valadas]

Gentlemen

My P/S ZF pump leaks hydraulic fluid at the top plug.  The one with a big
cross on top.  I have replaced the O-ring three times, but it keeps on being
destroyed under pressure.  Obviously I do not have the correct O-ring.  I
didn't want to buy the ZF rebuilt kit with a zillion rings since I'm not
planning on rebuilding the pump. Does anyone know where the proper ZF ring
for the top plug can be purchased by itself?

TIA

Lino
93 S4