Brake fluid exposure to air

[efnfast]

Why do most people freak out about exposing brake fluid to air (e.g., have 1/2 container of brake fluid left over, don't save for 3-4months incase you need it but immediately chuck it because it's been exposed to air). I know it's hygroscopic, but master cylinders are vented to atmosphere (e.g., both my Wilwoods and Tiltons have a straight-through hole in the vent cap) .... am I missing something? How is one activity "bad" (storing a bottle of previously opened brake fluid) but another (having the master cylinder vented to the air) "not"?

[rich grsc]

To all those throwing away 1/2 containers of brake fluid, I can send you my address, I will take them. It is virtually impossible for brake fluid to absorb moisture in a sealed container. Master cylinders open to he atmosphere is a different situation though. Over time the fluid will become contaminated.

[efnfast]

To all those throwing away 1/2 containers of brake fluid, I can send you my address, I will take them. It is virtually impossible for brake fluid to absorb moisture in a sealed container. Master cylinders open to he atmosphere is a different situation though. Over time the fluid will become contaminated.

Yes, but if brake/clutch fluid reservoirs are vented to atmosphere what's the big deal? Obviously the fluid absorbs moisture and needs to be replaced over time, but I don't undertsand why people get all freaked out about brake fluid being exposed to air when the reservoir in your car is naturally exposing it to air.

[kiki231]

actually the worry is that it will absorb the water in the air, as opposed to the air itself. As it absorbs the water , its boiling point drops which in turn reduces its effectiveness. The fact that the fluid absorbs water (hydrophilic) is a good thing, since pools of water in a metal system can cause other more dangerous issues. I still keep fluid after i open it. If it starts to turn a little brown, then i throw it away. That being said, i work on so many vehicles, and go thru so much fluid, that has never happened.
THe tiny vents in the rez serve to allow the air to equalize in the tank as the fluid displaces pad loss. The tiny bit of air poses a risk, and fluid eventually goes bad, but better than having it wide open.

[michael everson]

Actually most masters are not open to air. The hole is so that the seal can move as the fluid is used.
Mike

[Bob Cowan]

It is virtually impossible for brake fluid to absorb moisture in a sealed container.


Actually most masters are not open to air. The hole is so that the seal can move as the fluid is used.
Mike

Both of these statements are true. Ideally, water will not be absorbed into the fluid in the brake system or the plastic bottle because they are both sealed. In reality, neither are perfectly sealed. So some water will get in, no matter what you do.

I have no problem with a bottle siting on the shelf for a while. But I live in a dry climate, and it doesn't sit there for years.