Check out any of the twenty-two Fr. Brake Master Cylinder (st1300a) components enlisted.
In this schematic the "Washer,oil Bolt" is used most often.
The most reasonably priced component used on the St1300a Paneuropean 2003 (3) Australia / Abs Fr. Brake Master Cylinder (st1300a) is the Screw,flat,4x12.