Brake fluid attracts water out of the air. I think there is a synthetic fluid that would work better in our Jeeps and I am going to look into that when I redo my brakes.
I think it may be a good idea to flush your system with new fluid and replace or clean the cap. Is the reservoir rusted inside?
Carl, as someone who went that route I would recommend against it. When I replaced the entire brake system on my jeep 5 years ago I opted for DOT5 figuring that since it sat in the cool garage for 6 months out of the year, that was the way to go. After 5 years, if I had to do it over again I'd go with DOT4.
For starters, try pouring DOT5 into that MC without making any bubbles, what a PITA! I finally ended up using a small baster like an eye dropper. Second, the brakes do have a spongier feel to them and three, there's the possibility of moisture settling somewhere in the system and causing a rust pocket. I just may flush the system in the Spring and convert back to DOT4. �Just my $.02.
Larry