Ron,
One more thought: If you have a parts manual for your plane, it'll have an illustrated break down which shows you what you dealing with regarding the brake master cylinders including the part number of the o-ring that you'll need to replace.
You may (probably) will find it very helpful and I think there is a link floating around here where you can download free parts and service manuals. Someone smarter than me will need to jump in and post the links.
