reprinted from Motor Trend, December 1990 in the March 1996 RootesReview
Many questions have arisen about the use of silicone (DOT 5) brake fluids in their cars. I spotted this article in Motor Trend explaining the advantages and disadvantages in their use. As you read this, you will get some idea of what is the preferred use. From normal to extreme uses, such as when Terry Taylor uses his car in all different speed events, use only what the factory recommends, Girling (DOT 4) brake fluid. For uses, such as Rick McLeod’s Proto-Type Mk II on a limited basis, the silicone (DOT 5) is acceptable. In short, if you plan on only using your car for show and go, use the silicone.
The following is reprinted from MOTOR TREND Dec 1990 BETTER BRAKE FLUIDS
Brake fluid is brake fluid, right? Not exactly. Recent years have seen many advances in brake fluid technology, including the introduction of silicone-based fluids. Unfortunately, upgrading to high-tech braking isn’t as simple as swapping one type of fluid for another. The Department of Transportation (DOT) has for some time issued a set of specifications for brake fluid types. The more common glycol-based, or “conventional” fluids fall under DOT 3 and DOT 4 specifications. DOT 3 has a minimum dry boiling point of 401 degrees Fahrenheit. Silicone fluids fall under DOT 5 with minimum dry boiling point of 500 degrees.
Boiling points are important because, if brakes heat up and brake fluid boils, small gas bubbles are created that get trapped in the system, causing a spongy pedal. In a typical road car, conventional fluid boiling points diminish with time, with vehicles operated under damp conditions noticing a rapid deterioration of boiling point in as little as six months. While braking systems normally function with a percentage of moisture in the system, this moisture content increases over time, diminishing braking performance. That’s why “wet boiling point” specifications are also provided. These wet boiling points are DOT 3 (284 degrees), DOT 4 (311 degrees), and DOT 5 (356 degrees). The wet boiling point of DOT·5 silicone brake fluid is significantly higher than its glycol-based counterparts because silicone does not absorb moisture, while glycol fluid does. In addition to decreased braking performance, a number of internal brake components can also be damaged when fluid is intermixed with moisture. Components like disc brake pistons can corrode beyond repair, creating a dangerous situation and an even more expensive repair bill.
So why don’t OE manufactures make the switch to silicone based brake fluids? While silicone fluids do not absorb moisture (and unlike glycol-based fluids, won’t harm paint finishes if spilled), they do have a downside, including slight compressibility under extremely high temperatures, resulting in a spongy pedal. Silicone fluid is also affected by atmospheric pressure. When a silicone-equipped vehicle is driven in high-altitude conditions, 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 (EPR). Some silicone fluids are not compatible with these EPR components, causing them to expand.
Always use a brake fluid recommended by the motor vehicle manufacture. Never mix brake fluid brands or types, and make every effort possible to keep contaminants out of the brake fluid and braking system. Once the protective seal is broken on a can of brake fluid, use it immediately. Seriously consider the use of silicone brake fluid for cars that are only driven periodically, such as collector cars and antiques. And finally, consider flushing out the old glycol fluid in your car’s braking system, replacing it with fresh DOT 3 or DOT 4 fluid on a regular basis.