The o-ring that goes between the plastic reservoir and the metal body on 1977 and later front brake master cylinders is 39.7 x 3.5.
I covered this pretty thoroughly in this thread:
http://www.xs650.com/threads/front-brake-m-c-reservoir-o-ring-part.50478/
Jump to post #13 -- there is a lot of other good info in post #13 of the above thread regarding that o-ring.
I covered this pretty thoroughly in this thread:
http://www.xs650.com/threads/front-brake-m-c-reservoir-o-ring-part.50478/
Jump to post #13 -- there is a lot of other good info in post #13 of the above thread regarding that o-ring.