brake fluid overflowing problem

[barryv]

i took my car out for a ride yesterday for approx 50 miles .when i got home i checked engine  oil level.i noticed master cylinder looked a little wet.i took cover off and brake fluid started to overflow.does this mean i have to much fluid in system and when it got hot it over flowed when cover was removed or is there another issue,thanks

[MEK-Dangerfield]

Do you have an aftermarket cover on the master cylinder? If so, it may take some tweaking of that metal bar to make it seal right.

[barryv]

no,its an original master cylinder type,thanks

[Changin Gears]

It may mean you have air bubbles in your brake lines and need to  bleed them.  Brake fluid doesn't expand very much with heat but air does.

[barryv]

its kind of strange that this happenned. i have had brake system untouched for about 10 years except adding a drop of brake fluid,thanks

[Aussie Challenger]

  All fluid will expand when heated, if the master cylinder was filled to the top, not leaving an air gap it will over flow when lid taken off. When brakes are applied there is a small amount of fluid which will squirt up within the reservoir. If the top gasket doesn't seal properly the gasket will leak making the outside of the master cylinder damp/wet.

[Bullitt-]

This come to mind...
    10-year old brake fluid could have absorbed moisture making it susceptible to boiling, recommended change interval is about 2-years.

http://www.shotimes.com/SHO3brakefluid.html
    The amount of moisture in brake fluid definitely affects its performance. The big problem is it is absorbs moisture quickly. Over a relatively short period of time brake fluid will absorb moisture from the air. SAE field tests have shown that the average one year old car has 2% moisture in the fluid. A random test of vehicles in the U.S. showed an average water content of 2.6% for vehicles with an average age of 8 years. And 25% of these vehicles had water content greater than 4%.

As water content in brake fluid increases over time, the boiling point decreases. Fluid with a reduced boiling point (or high water content) can create vapor by boiling in the caliper, or wheel cylinder. The result is sudden brake failure

[alphabuck]

i saw firsthand this past weekend the moisture absorbing properties of dot 3 or 4 brake fluid.  i just did my whole car front to back with new shoes and wheel cylinders and everything else.  in the process of swapping out the master cylinder i managed to spill a mess of fluid on my fender.  not caring about saving any paint i just wiped it off.  the next morning the area i had wiped off was just drenched.  in an otherwise completely dry storage area it looked like someone had sprayed down the brake fluid stained area with a spray bottle.