Originally Posted by Cogswell
Looks to me like there are two options for the rear master cylinder, and two options for a rebuild kit depending upon date of manufacture.
Maybe you have the incorrect one for your bike ? Check the part number against the fiche and let us know.
I'm sure I've got the right part, the newer 1981-1984 piston is smaller by 1.58mm
It appears the cup style seals are a superseded design over the old o-ring style seal.