Why brake fluid?

September 17, 2008 2:25 AM

Why brake fluid? It's horrible stuff; volatile, poisonous, corrosive, generally unpleasant. So why do we use it instead of using a (relatively) more beign hydraulic fluid?
posted by BadMiker to Travel & Transportation (6 answers total)

Brake fluid is expected to hold up under fairly extreme conditions - extremes of temperature, long-term use without degrading, it's hygroscopic by design so that water that might corrode brake components isn't left in contact - there are some more bio-friendly fluids that are used in certain situations, but the current fluid is most likely the best performing (and cheapest) available.
posted by pupdog at 2:42 AM on September 17, 2008 [2 favorites]

pupdog is generally right, but it's also worth pointing out that given the relatively low risk that most people have of being exposed to brake fluid on a regular basis, the risk is generally worth the benefits. Same goes for most of the other reasonably exotic fluids in your car, e.g. everyone knows antifreeze is poisonous, but we use it anyways.
posted by valkyryn at 4:15 AM on September 17, 2008

What pupdog said with the addition that a really high boiling point is the key with brake fluid versus other types of hydraulic fluids.

DOT 3 brake fluid, the most common kind, isn't really all that nasty. It's essentially mineral oil. DOT 5 is silicone based, so no problems with water absorbtion, but it's a bit squishy under the foot.
posted by three blind mice at 4:24 AM on September 17, 2008

Brake Fluid can also take paint off of any metal surface if you feel the need to find out.
posted by captainzero at 6:38 AM on September 17, 2008

Not all hydraulic fluids are "benign". The fluid used in older Hurst rescue tools is an aircraft type and very irritating to the eyes, skin and mucous membranes. I know this first hand thanks to a face full of it trying to deal with a hydraulic hose problem (goggles saved my eyes but not my nose and mouth). Their newer fluid is mineral oil-based and not quite as bad.
posted by tommasz at 7:23 AM on September 17, 2008

DOT 3 brake fluid, the most common kind, isn't really all that nasty. It's essentially mineral oil.

No, DOT3 is glycol and glycol ethers, somewhat related to anti-freeze. It is not petroleum based. That is because mineral oils cause rubber gaskets, seals and tubing to swell.
posted by JackFlash at 10:26 AM on September 17, 2008