Brake fluids are hygroscopic (water loving), which means they absorb moisture from the atmosphere under normal humidity levels. I do not know what kind of maintenance you did with your brake system but if your brake fluid has not been replaced in a while now is the time to do it.
The same happened to me. Whenever you change your brake fluid make sure you do a couple of cycles to make sure you have fresh fluid in your system.

Good luck.
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Javy

88 red Montero with the Dakar Special Edition decals, 438,000 miles (As of 7/23/2012) some occational blue puffing but still strong and counting!!! I am going ppsshhh (turbo), Stay tuned =) ...