Evaluating Brake Fluids DOT 3 4 5 - What's Hot And What's Not

Understanding Your Choices

Wayne Scraba Jan 1, 2005 0 Comment(s)
Corp_0501_01_z Prestone_brake_fluid Bottle 1/1

Feel like opening a can of worms? Engage in a debate on brake fluid. Why? Brake fluid is brake fluid, right? Not exactly. Over the past several years, many advances have been made, not the least of which are the silicone-based fluids. Unfortunately, it isn't as simple as swapping one type for another.

The Federal Boiling PointFor some time, the Department of Transportation (DOT) has issued a set of stringent specifications for brake-fluid types. The more common glycol-based or conventional fluids fall under DOT 3 and DOT 4 specifications. On the other hand, silicone fluids fall under DOT 5. In a typical race or high-performance car, the conventional fluid boiling point diminishes with time. How long does it take? We don't have statistics for Corvettes, but in most passenger cars operated under damp conditions, rapid deterioration of brake-fluid boiling point can occur in as little as six months.

Obviously, many Corvettes don't see as much rain as daily drivers, but brake fluid is definitely affected by time and weather. That's why some race-car brake manufacturers recommend flushing the system with fresh fluid on a regular basis. Some road racers change fluid after each event.

What's the big deal about brake-fluid boiling points? One of the most critical factors of a hydraulic brake system is the dry and wet boiling point. If the fluid boils, small gas bubbles develop and they're trapped in the system. Gas bubbles are compressible, and the brake pedal becomes spongy.

Wet Vs. Dry Boiling PointsMost likely, a street-driven passenger car is plagued by wet boiling points. Normally, the brake system functions with a percentage of moisture in the system. Over time, the moisture content increases and, as a result, brake performance decreases. That's why wet boiling point specifications are provided by the DOT.

DOT 3 : 284 degrees F
DOT 4 : 311 degrees F
DOT 5 : 356 degrees F
DOT 5.1: 375 degrees F

(Dot 5.1 is often difficult to locate in North America.)

As you can see, the wet boiling point of silicone brake fluid is significantly higher than its more common glycol-based counterparts (we'll get to 5.1 fluid later). That's because silicone fluid doesn't absorb moisture, while glycol-based fluid does. A can of conventional glycol-based brake fluid almost always has a warning to use the fluid immediately because of moisture absorption. Once the seal is broken, moisture attacks the fluid in the can.

Moisture And Brake Fluid Don't MixWater in the system can attack a number of internal brake components, which decreases brake efficiency. Disc-brake pistons can corrode beyond repair, which creates a dangerous situation and an expensive repair bill. It's a good idea to use the brand of brake fluid in the system (or the brand you're planning to use) as a lubricant when assembling brake calipers and components.

Why don't OEM manufacturers and racers make the switch to silicone-based brake fluids? They have some good qualities: They don't absorb moisture and don't harm paint finishes as quickly as conventional fluids if spilled. Unfortunately, they aren't perfect. One problem is slight compressibility under high temperatures. If the brake system is exposed to very high temperatures, the silicone fluid can compress slightly, which causes a spongy pedal.

In addition, silicone fluid is affected by atmospheric pressure. When a vehicle with this fluid is driven in high altitudes, the fluid can expand significantly, again contributing to a spongy pedal. Further, many of the rubber components used in brake systems are manufactured from ethylene propylene rubber. Some silicone fluids aren't compatible with the EPR, causing them to expand.

What's Hot?Question any seasoned racer about brake fluid, and one of the first things he'll mention is Ford High Performance Brake Fluid (sometimes referred as Ford Heavy Duty Brake Fluid). This is usually the fluid of choice, but let's back up. The good racing brake fluid available today is Castrol SRF. It's a somewhat rare super fluid designed primarily for racing, but most people don't buy it because of the high cost. It can be purchased from a Ford dealer under part number C6AZ-19542-AA. The packaging states, "High Performance Dot 3," and the cost is considerably less than the Castrol-packaged fluid.

Castrol LMA is another good-quality brake fluid. Castrol GT LMA is the more pedestrian fluid primarily used by the British-car crowd because it doesn't eat the oddball British rubber that might be in their cars. It's rated DOT 4, and may be used in all cars, with the exception of some German makes with different fluid requirements. Castrol GT LMA is adequate for most cars used in autocross, but a fluid more resistant to boiling is likely necessary for racing or track events.

Castrol LMA rejects moisture and may be kept in your brake system for several years. LMA stands for Low Moisture Absorption. It's sold in plastic containers that don't have a long shelf life. Don't buy a large quantity because moisture can make its way through the plastic containers.

Ford Heavy Duty DOT 3 is inexpensive and popular among racers because of its excellent dry boiling point. It absorbs moisture quickly, but racers don't care because they change fluid frequently. It's sold in metal cans and has a long shelf life (provided the seal isn't broken).

Ford Heavy Duty is classified as a DOT 3 fluid. Its dry boiling point is 550 degrees F, while the wet boiling point is 284 degrees F. Castrol LMA is classified as a DOT 4 fluid. It has a dry boiling point of 446 degrees F and a wet boiling point of 311 degrees F.

Something New"Super" brake fluid or DOT 5.1 is now available (primarily in Europe). It has a dry boiling point of 518 degrees F and a wet boiling point of 375 degrees F. For the most part, 5.1 fluids are polyglycol ether-based, which is similar to DOT 3 and 4. The 5.1 fluids can mix with water and perform adequately. Polyglycol-type fluids are two times less compressible than silicone types, even when heated, which increases pedal feel. But not all 5.1 fluids are the same. At least one company (Motul) manufactures a silicone-based fluid that meets the DOT 5.1 specification.

Dot 5.1 is a relatively new brake fluid that's causing confusion among mechanics, which the DOT could have avoided by giving the new fluid a different designation. Some believe the 5.1 designation is a modification of silicone-based DOT 5 fluid. Calling it 4.1 or 6 might have been more appropriate since it's a glycol-based fluid like the DOT 3 and 4 types, not silicone-based like DOT 5. Some companies are marketing a similar new fluid, which is often called Supreme DOT 4.

The 5.1 fluids are much like high-performance DOT 4 fluids, rather than traditional DOT 5 silicone brake fluids. For example, DOT 5.1 provides superior performance over many of the other brake fluids we've discussed. It has a higher boiling point, either dry or wet, than DOT 3 or 4. In fact, its dry boiling point (518 degrees F) is as high as most racing fluids, and the wet boiling point (375 degrees F) is higher than most. In addition, DOT 5.1 is said to be compatible with all rubber formulations.

All the news is not good, though. DOT 5.1 is a non-silicone fluid that absorbs water. It also eats paint, like DOT 3 and 4 fluids. DOT 5.1 isn't cheap or easy to find.

What should you use on a street-driven Corvette? We like Castrol LMA or any non-silicone-based 5.1 fluid. If you race your Corvette and change fluid regularly, Ford Heavy Duty fluid is a good choice.

Simple Fluid RulesIs there more? You bet. Always use a fluid recommended by the brake or vehicle manufacturer. Never mix brands or types and make every effort to keep contaminants out of the fluid (as well as the brake system). Once the protective seal on a can is broken, use it immediately. Never reuse brake fluid that was bled from a system.

Silicone brake fluid can be used in cars that are rarely driven (collector cars, antiques, trailer queens). To maintain the health of your system, flush out the old glycol fluid on a regular basis, and replace it with fresh, high-quality fluid. Your brake pedal will be much happier.

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