The most reasonably priced component used on the Dr650se 1998 (w) (e02 E04 E17 E18 E22 E24 E34) P37 Rear Master Cylinder is the Nut.
On this parts fiche the Washer(10x15x1.5) is the most specified component.
A sample from the products for the Rear Master Cylinder include: Guide,rr Brake, Hose,reservoir Tank, Cap, Guide,rr Brake Hose.