QI like to think of myself as one of those street rodders who keeps his car in excellent condition. I do all the preventive maintenance necessary each winter and have had the car inspected by the NSRA safety team at the Street Rod Nationals and everything was good except for the parking brake, which I’ve since repaired.
My car is a ’37 Ford, it has disc brakes up front, a Ford 8-inch rear with drum brakes in the rear, and a Ford dual chamber disc/drum master cylinder with a dual-diaphragm booster. The brake lines are 3/16-inch stainless with braided stainless flex lines front and rear, with a 10-pound residual pressure valve in the front line.
While the brakes work perfectly, when I did my winter maintenance, I discovered the brake fluid was extremely murky, almost muddy looking. I don’t remember it looking that way the last time I checked it, but since the master cylinder is under the floor it does not get checked as often as it should.
Obviously the brake fluid in my car needs to be changed, but what is the life expectancy of brake fluid and what’s the best way to know if it needs to be changed?
Kyle Kraft ViatheInternet
ABrake fluid is one of those things that everyone more or less ignores. However, due to contamination from moisture it should be changed when necessary.
According to the brake experts at Wilwood there are many ways for moisture to enter a brake system, including condensation from regular use, washing the vehicle, and humidity, and there is not much that can be done about it.
DOT 3 is the most common type of brake fluid used in domestic cars and trucks. According to the Society of Automotive Engineers (SAE), DOT 3 can absorb 2 percent of its volume in water every year. Over enough time excessive moisture will cause corrosion in the brake system and can lead to issues like vaporlock or a spongy pedal.
DOT 4 is formulated for use by all vehicles, it has a higher boiling point than DOT 3, and it does not absorb moisture as fast. DOT 4 and DOT 3 are interchangeable, however it’s best to avoid adding DOT 3 fluid to a system that already uses DOT 4. (It’s the preferred type of fluid used for street and high-performance applications.)
DOT 5 (often referred to as synthetic brake fluid) is silicone-based, which means it does not absorb any moisture. Many street rodders use synthetic because it’s not corrosive to paint or other brake components, which makes it great for preserving classic cars for long periods of time. But there are a few drawbacks to siliconebased fluids. They expand more when compressed, which can make the pedal feel spongy, also DOT 5 fluids cannot be mixed with any other type of brake fluid. DOT 5 will usually have a violet tint in color to distinguish it from DOT 3. Wilwood does not recommend using DOT 5 fluid in any racing applications. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel.
DOT 5.1 is a non-siliconebased polyglycol that has a boiling point over 500 degrees. Unlike DOT 5, DOT 5.1 can be mixed with DOT 3 or DOT 4. Also, DOT 5.1 will usually have the highest rated boiling point, which is best recommended for severe-duty and high-performance applications. AFCO racing offers the Ultra
HTX that exceeds DOT
5.1 requirements with a boiling point rated over 600 degrees.
Due to the extreme operating temperatures of a high-performance brake system, standard, off-theshelf brake fluids are not recommended. Of critical importance in determining a fluid’s ability to handle high-temperature applications is the
Dry Boiling Point and compressibility.
The Dry Boiling Point is the temperature a brake fluid will boil in its virgin non-contaminated state. The highest temperature Dry Boiling Point available in a DOT 3
fluid is 572 degrees F. The Wet Boiling Point is the temperature a brake fluid will boil after it has been fully saturated with moisture. The DOT 3 requirement for wet boiling point is a minimum temperature of 284 degrees F.