On this parts fiche the Washer(10x15x1.5) is the most specified component.
Some items from this specific schematic: Cap, Union, Diaphragm, Hose,reservoir Tank.
The most reasonably priced component used on the Gs500e 1999 (x) (e02 E04 E22 E24 E25 E34 E37) Rear Master Cylinder is the Pin.