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Brake Fluid

First, this is based on the premise that you know something about brakes. If you don't, this is a bad place to start because all I intend to write about is brake "fluid". Of course, I've been known to sway from my intended path with some degree of regularity.

Brake fluid is a hydraulic fluid used to transfer force under pressure from a master cylinder, through the brake lines, to the calipers. If you have drum brakes, then instead of calipers, you've got wheel cylinders. At any rate, this works because brake fluid doesn't compress. Let me clarify that; it compresses some, but not so you'd notice. Well, fresh brake fluid doesn't noticeably compress. 10-year-old, water-logged brake fluid compresses, but that's why you're supposed to change your brake fluid every couple of years.

So how does water get in brake fluid? Brake fluid is hygroscopic, which means it absorbs moisture from air. That's right, "air", and it's absorbed pretty quickly. SAE field tests have shown the average one year old car has 2% moisture in it's brake fluid. As the amount of water in brake fluid increases, the boiling point decreases. As the boiling point is reduced, so is performance. That's why all brake fluid has a DOT rating. Here are the DOT ratings:

When brake fluid contains 3.7% water, it's considered "wet". Under normal circumstances, wet fluid will only give the brakes a spongy pedal feel and corrode the braking system to the point where something expensive starts leaking. On track days, things are a little different. There, brakes get very hot and old brake fluid can easily boil. When brake fluid boils, it changes from a liquid to a gas. Gas is compressible. That compressible gas causes long pedal travel followed by complete failure when ignored for "just one more lap".

Before you glance at the chart and make a decision, there are two "don'ts". Don't use DOT2 or DOT5.

• The DOT2 spec for anything regarding brakes is obsolete. If you have any DOT2 in your garage, do yourself a favor and toss it. All DOT2 is good for is rotting out brake systems

• DOT 5 is a silicone brake fluid and is designed for vintage cars whose brake parts are no longer available. Silicone brake fluid (DOT 5) has a few problems: (1) It's hard to pour without introducing bubbles. The bubbles will cause soft pedal feel. (2) Aside from that, it's slightly compressible, so even without the bubbles, it gives lousy pedal feel. (3) It's not compatible with regular brake fluid. A complete flush of the entire brake system must be performed before using silicone fluid. That's a complete flush as in running alcohol through the system, then disassembling and cleaning all the components. (4) It doesn't absorb water, so water will puddle in the lower parts of the system where it will devour parts. (5) It's about twice as expensive as DOT3 or 4.

That leaves 3,4, and 5.1. Forget about DOT3. It absorbs moisture a lot faster than DOT4. So the choices are DOT4 and 5.1. Despite the DOT ratings, all fluids aren't created equal. Some have much higher boiling points. Here are some popular brake fluids and their boiling points:

5.1 is clearly the superior fluid, but it’s not on the list. That’s because I have trouble finding a supplier. So for me, it’s DOT4. Castrol SRF is hands down the best fluid, but it’s about $75 a liter.

Personally, I rotate between ATE Blue, and ATE 200 every other year. They’re both the same fluid and cost about $12 per liter. The Blue has blue dye, and the 200 is a natural gold color. The reason I rotate between those two colors is because when I flush the system, I can see that I'm done at one corner because the color changes. Motul makes a superior product, and it’s available at most motorcycle shops, but I like the blue dye.

Whatever you choose, change the fluid at least every other year. Brakes components are expensive.

Here are some closing notes:

• A brake fluid’s dry boiling point is more important than it’s wet boiling point on a race car.

• In passenger cars, brake fluid changes are usually skipped. This makes the wet boiling point more important than it’s dry boiling point.

• Racing fluid typically exceeds DOT 3, 4, and 5.1 dry boiling point specifications, but falls short of the DOT4/5.1 wet spec.

• Never use silicone based fluids in anything other than a classic.

• Using race spec brake fluid will increase performance of the braking system, but it should be changed often.

• Never mix types or brands of brake fluid.

• If fluid remains in container, it should be tightly sealed and used in a short period of time.

• Mercedes-Benz factory fill/recommended brake fluid is DOT4 Plus. It’s part number 000 989 08 07 01, which meets their spec of Blatt 331.0 This is rumored to made by Exxon-Mobil and is rated at 500° F/356° F.