Dave,
Routine maintenance requires checking the level of brake fluid in the master cylinder on a periodic basis. Even a trailer queen requires inspection, at least once a year, to be sure a leak has not occurred, dropping the level below the inlet port and producing a dangerous situation. Observing the fluid also indicates untoward corrosion or water in the fluid. I would respectfully suggest you change your carpet arrangement so that you can access the master cylinder through the floorboard as it was designed. You don't want to be guessing or hoping everything is OK.
There is no simple method to keep from overfilling. You want the level to be below the bottom of the cap, which has a built in baffle. Having said that it is difficult to gauge the level when the fluid is nice, clean and crystal clear. Personally, I wouldn't sweat that amount of seepage, especially after 5 years.
But I would sweat it if I hadn't seen the inside of my master cylinder for five years.
Regards,
Kent
