Synthetic brake fluid?

Possibly low-hygroscopic DOT 5.1 or 4? I've seen some claims of brake fluid that can go 5-10 years without replacement, but I'm highly skeptical. There are some super exotic racing brake fluids like Castrol SRF, which is about $70 for a 1 liter can.

BTW - nearly all modern brake fluids are "synthetic" save some exotic systems requiring mineral oil based fluids. Using the term "synthetic" to describe brake fluid is nothing more than a marketing gimmick. Even rather ordinary Prestone DOT 3 is now being marketed as "synthetic".

Well, what I do know, after lots of research, is that you actually want your brake fluid to absorb water. That's a good thing. So, if your brake fluid is doing its job you need to replace it on a regular basis.

marketing

It's two things really. You want it to absorb water that ends up in the system, so that there aren't small pockets of water that cause rapid localized corrosion. Silicone doesn't mix with water and water that ends up in the system ends up in the low points. Silicone doesn't attract water though.

However - ordinary glycol-based brake fluids are hygroscopic, meaning that they will absorb (actually attract) water vapor from air. Different brake fluid formulations are better or worse in that regard. I remember hearing about a demonstration where someone took a sealed bottle of brake fluid in a humid climate at a racetrack near Atlanta. He then poured the fluid into a shot glass up to the top. After an hour, the fluid had spilled over simply because it attracted and absorbed moisture from the air.

Apparently ordinary brake fluids have a certain shelf life because a small amount of moisture can permeate the plastics (or even the plastic seals of metal cans). Once opened, brake fluid should be used quickly - maybe less than two weeks to top off after a brake job. No brak system is perfectly sealed, so moisture will be attracted into the system by the fluid.

So, what would you put in your car? Synthetic or DOT 4? Why?

Thanks.

Sheldon snipped-for-privacy@sopris.net

I've used various DOT 4 brands, and had never been particular about brands. I buy the fluid and take it to the shop. Among the brands I've used are Prestone DOT 4, Castrol GT/LMA, Pyroil DOT 4, and Valvoline SynPower. I just brought my car into the shop today, and left 5 .25L cans of Motorex DOT 5.1 fluid for the shop to install. I also have three bottles of Motul DOT

Also - almost all normal brake fluids are "synthetic". DOT 3, DOT 4, DOT 5, DOT 5.1. The only brake fluids that aren't would be some exotic mineral (petroleum) oil based ones, used in some Citro=EBn or older British cars.