I pulled the cap off a couple of days ago and noticed a bit of condensation on the underside of the cap.

What does this mean? I thought brake fluid was supposed to be free of moisture or water?

Also does anyone know where the air vent is on the cap?

 

Brake fluid is ideally free of water, however it is hygroscopic (attracts and holds water). Once saturated, it might allow some water to vaporize and condense on the cap.

For this reason, it's recommended to replace brake fluid every 2-3 yrs (check owner's manual) to avoid corrosion of brake lines (steel).

good luck

 

The air vent is a tiny hole in the cap that's usually difficult to find.

Water is completely soluable in brake fluid, and here's why...
If it stayed separate (like water & oil) then the water would migrate down to the brake calipers. Then the boiling point would be 212.F which is much too easy to boil. With some amount of water dissolved in the brake fluid, it's boiling point is still much higher than 212.F.

Problem is; once water gets in there, it makes the fluid corrosive.

 

Hate to revive an old thread but is it safe to just change the fluid in the reservoir? I have half a can of brake fluid that I just opened today so it's not "old" enough to have attracted moisture I think. I'm thinking I want to just turkey baster drain the reservoir and throw this in there. I know it isn't going to change all of the fluid but at least it changes what was in the reservoir.

Safe to do? Drain the reservoir, clean it up a little and throw the new fluid in there.

 

It won't hurt anything. Any water in the brake fluid will be absorbed by fresh fluid.