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Which Brake Fluid?

A Fluid Answer

Curt Schneck; via I have a ’70 Pontiac LeMans Sport and I am adding a disc brake setup that I built with stock spindles, stock calipers, stock master cylinder, cross-drilled rotors, and performance pads. I am also, at the same time, replacing all the brake lines with stainless steel lines, stainless steel braided hoses, the proportioning valve, and new, rear-wheel cylinders for the rear drums. I would like to know what brake fluid to use when doing this conversion: DOT 3, 4, or 5? The whole system will be new, so I don’t have to worry about purging the system to use silicone brake fluid. This car sees the street and really does not see the track. I know DOT 3 brake fluid absorbs moisture. Since I live in the northeast where the car sits in a non-climate-controlled garage, is that an issue? What are the differences between DOT 4 and DOT 5? Are there any performance advantages/disadvantages between them, besides the boiling point? Keep up the great work guys—Car Craft is the only real-world car mag!

Jeff Smith: These are great questions, Curt. All common brake fluids are generally referred to as hygroscopic, which means they have a tendency to absorb water. That is why all master-cylinder covers use a rubber liner between the fluid and the outside atmosphere. It is also why the brake fluid container should always be stored with the lid tightly closed. What is less well known is that while stock flexible-rubber brake lines do a great job of keeping the fluid in, the brake fluid can still absorb water through these lines, as implausible as that may seem. How the car is used helps to determine the kind of brake fluid you should choose. Let’s start with the basics. The difference between DOT 3 and DOT 4 fluids is, as you suspected, more than just the boiling point. The DOT standard for a wet boiling point is 3 percent water. This may not sound like much, but check out these boiling points as listed on Amsoil’s website in the chart below. Note how both the DOT 3 and DOT 4 Amsoil fluids drop a greater percentage of the boiling point compared with the DOT standards when subjected to 3 percent of water mixed with the fluid. This is a typical negative to pushing the dry boiling point higher. DOT 3, 4, and 5.1 fluids are polyglycol ethers, while DOT 5 is silicone based. As you’re probably aware, normal brake fluid is an excellent paint remover, but silicone-based fluid is not, which makes choosing DOT 5 a great idea for show cars. The downside to DOT 5 fluid is that it is compressible. That means as the fluid gets hotter, the brake pedal becomes spongy, which is not what you want, especially if this fluid were used in a car driven on road courses or other situations in which the fluid could see extreme temperature. The plus side to DOT 5 silicone fluids is that they have a low affinity for water, which makes them much better for use in cars like yours that are subjected to humidity. However, water can still exist in a brake hydraulic system with silicone fluid; it tends to collect at the bottom of either the brake master or calipers, where it may be troublesome to remove. It appears that DOT 5 might be a better fluid for a car like yours that is parked for long periods of time.

For performance cars on which the brakes are abused at track days or autocross events, the best choice is the DOT 4 fluid. We’ve used the Amsoil DOT 4 with no problem in our ’65 Chevelle road-race car, but we change the fluid after two events, completely flushing the system front and rear. We learned that Raybestos now offers a new Super Stop Super High Performance DOT 3 brake fluid with a rated dry boiling point of 550 degrees F that sells for $5.99 per can.

More Info

Amsoil; Superior, WI; 715/392-7101;

Phoenix Systems; St. George, UT; 435/673-0777;

Raybestos; McHenry, IL; 815/363-9000;

DRY 401 535 446 580 500 518
WET 284 313 311 410 356 374

*All temperatures are listed in degrees Fahrenheit

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