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Brake Fluid Explained
Abbreviated from a TRD article
James Walker
a brake specialist who has worked for Delphi, TRW, GM, Bosch, and Ford. 
He is currently an engineer for Carr Engineering and a freelance writer.

This is just a brief run down on Brake Fluids, their designations and applications and maintenance.

First, let's understand DOT ratings. DOT stands for Department Of Transportation. The DOT (sometimes referred to as the Dotties) write characteristics for brake fluid. They do not say what it's composition is, just what it should do. If you can get maple syrup to meet the specs, then it is good. All of the DOT ratings must meet 4 specs:
1. It must not solidify. There is no low end number for this but since -40F is fairly common in the USA you know all fluids must function below that.
2. It must not vaporize. Again, there is no specific number given by DOT, but since 400F can easily occur at the cylinders, we can expect a fairly high evaporating number.
3. It must be compatible with rubber seals.
4. It must not be excessively compressible.

So the one fluid that meets all of the above and is yet reasonably easy to make and cost-effective is a glycol-ether based fluid. However, glycols have one very undesirable side effect, they love to absorb water. This is why brake fluids come in sealed containers. Once it has been opened, the water absorbing process begins. In a sealed system, like our brake lines it's a small problem. But for a half empty container without a seal sitting on the shelf, the amount of H2O grows rapidly. Even with the cap on tight, the container is "breathing".  Getting warm during the day and cooling off at night, air with moisture is seeping into the container and being absorbed by the fluid.

When you use this half full brake fluid, you are adding moisture to your braking system and rust will start to occur. It also lowers the boiling point of the fluid and raises the freezing point. This is where it gets kind of weird. The colder it gets, the lower the ability of the glycol to hold water, so pockets or bubbles of water can form which then freezes. So the point is to buy smaller bottles of fluid to keep on hand and discard the remainder after it is opened.

Now, realistically, water is everywhere and will eventually find its way into everything, including your sealed brake system. The point being made here, is not to help it along any further. Besides, if you flush your system at regular intervals, the old fluid will also flush any water it has absorbed. Discard half full shelf bottles.

So as brake systems got more intense, DOT started upgrading their specs. The oldest spec you can still buy is DOT 3. The Dotties also decided to list the specs as "dry", that is 0% water content, and "wet"¯, which is 3.7% water content. Beyond 3.7% water content is considered unusable.

DOT 3 – is still a basic glycol-ether fluid, but must a have a 0% water content boiling point of 401F, and a 3.7% water content boiling point of 284F. (As Mary Poppins would sing: "Just a little bit of water makes your stopping distance longer".)

DOT 4 - starts seeing the addition of borate esters which helps resist water. The 0% H2O boiling point of DOT 4 is to be above 446F, and the 3.7% boiling point is 311 F. Yet the borates will degrade with time in which they start absorbing water and make the boiling points go down below those of DOT 3. So it is recommended that you use DOT 4 only if you are religious about flushing and changing your brake fluids.

DOT 5 – Is the only really different brake fluid as it is Silicon based. It's 0% boiling point is 509F and it's 3.7% boiling point is 356F. Also, it is a thinner fluid making it more freeze resistant. So is this the Grail of brake fluids? Well, there's 2 downsides. The first is that because of it's larger molecules, air molecules can fit in between and resistance to compressibility goes down. Two is that all the fluids listed here DOT 3,4,5.1 are all intermixable. But DOT 5 stands alone, it cannot be mixed with any other fluid. Another advantage is that DOT 5 is less corrosive to paint than the others. Currently only one manufacturer supplies Dot 5 as OEM fluid: Harley.

Finally comes DOT 5.1 – So finally manufacturers came up with a glycol formula that matched the DOT 5 silicon based characteristics. Since the two were not compatible and DOT didn't want confusion, they called this new formula 5.1. Sometimes manufacturers refer to this as 4+ or Super 4. It is pricier.

There is no magical formula as to which is better. Somewhere between price and performance is what you will settle on. The two big notes here are flush your system at regular intervals as prescribed by the vehicle manufacturer and don't keep opened bottles for use later. Buy big bottles for the actual flush and keep a little sealed bottle for topping off if necessary.
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