Brake Down (pun)

• brake fluid wears out over time.
• brake fluid is probably the most neglected fluid in vehicles
• as brake fluid ages, the corrosion-inhibiting additives in it break down
• the earliest signs of chemical decomposition can often be detected within a year or so, or after 10,000 to 12,000 miles of normal driving
• Hard use that elevates brake temperatures and pressures can accelerate the rate at which the fluid additives break down even more
• As time goes on, heat and chemical activity continue to erode the fluid's ability to prevent corrosion
• Oxidation starts to eat away at metal surfaces creating dissolved acids and sludge that are carried with the fluid as it surges back and forth with every application of the brakes.
• Some of these contaminants are abrasive and increase seal, piston and bore wear in the calipers, wheel cylinders and master cylinder.
• As the fluid gets dirtier and dirtier, it accelerates mechanical wear in the brake system.
  

Moisture

• Moisture also creeps into the system to further add to the contamination problem
• Humidity in the air can be drawn into the fluid anytime the fluid reservoir is opened for inspection or anytime a brake hose, line or bleeder valve is opened when servicing the brakes.
• Even if the fluid reservoir is never opened, moisture will still infiltrate the system through microscopic pores in rubber brake hose.
• The rate at which this happens depends on the age of the vehicle and the type of brake hose used.
• Newer hose made with linings that are less permeable to moisture help extend the life of the fluid. Even so, moisture can still slip past piston seals in wheel cylinders and calipers.

• Glycol-based DOT 3 and DOT 4 brake fluids are both "hygroscopic," which means they attract water.
• Leave a bottle of brake fluid open for several days and it will pull moisture out of the air like a magnet.
• This is one reason why vehicle manufacturers use glycol-based brake fluid: it disperses moisture that enters the system throughout the fluid to dilute the contamination. This prevents moisture from forming puddles in calipers or wheel cylinders that could boil and cause pedal fade if the brakes get too hot. The downside is that moisture lowers the boiling temperature of the fluid, increases its viscosity and promotes internal corrosion. That's why various chemical additives are put into the fluid to help it fight corrosion and oxidation.
• All brake fluid must meet minimum performance standards established by the U.S. Department of Transportation (DOT). Most brake fluids usually surpass these standards, but it's important to know what the minimums are because of their potential impact on driving safety.
• For all fluids that meet the DOT 3 specification, the minimum "dry" (contains no water) boiling temperature is 401° F and the minimum "wet" (saturated with water) boiling temperature is 284° F.
• For all fluids that meet the DOT 4 specification, the minimum dry boiling temperature is 446° F and the wet boiling temperature is 311° F.
• How do these numbers compare with most new brake fluids? For many years, Ford has used a special "high temperature" DOT 3 fluid with a dry boiling temperature of 580° F. General Motors, Chrysler and most "heavy-duty" aftermarket fluids typically have a dry boiling temperature of at least 475° F and some go as high as 550° F. By comparison, most DOT 4 fluids start out with a dry boiling temperature of 509° F or higher.
• As you can see, most new fluids far surpass the minimum standards required by the DOT. This provides an extra margin of safety to minimize the risk of fluid boil and pedal fade in severe braking situations when the brakes get really hot. But, moisture contamination can quickly reduce the margin of safety.
• According to some studies that have been done, brake fluid typically absorbs about one percent or more moisture per year of service life. Many two-year-old vehicles have as much as two to three percent water in the brake fluid. Imagine how much water must be in some vehicles that are six, eight or 10 years old and have never had the fluid changed!
• As the fluid becomes contaminated with moisture, its boiling temperature drops. Only one percent moisture can lower the boiling point of some DOT 3 fluids down to 369° F. Two percent water can push the boiling point down to 320° F, and three percent can drag it all the way down to 293° F - which is getting dangerously close to the minimum DOT requirements.
• Why don't the vehicle manufacturers simply switch to DOT 5 brake fluid that is silicone-based and repels moisture? One reason is that silicone brake fluid is very expensive compared to glycol-based fluids. Another is that silicone fluid contains more dissolved air and aerates more easily when pumped rapidly through small orifices that can increase pedal travel and reduce pedal firmness (a spongy pedal). Because of this, most vehicle manufacturers warn against using DOT 5 brake fluid in any vehicle equipped with ABS.
• DOT 5 also cannot disperse moisture that enters the system throughout the fluid like DOT 3 and DOT 4 fluids. Any water that does get inside tends to "slug" and settle to the lowest point in the system, which is usually the calipers and wheel cylinders. Slugs of pure water are not something you want inside a brake system because they concentrate corrosion and increase the risk of pedal fade if the water gets hot enough to boil (it only takes 212° F to turn liquid water into steam).
• Where DOT 5 fluid works best is in vehicles that sit for long periods of time (antique and classic vehicles that are stored during winter months or in a museum), or in vehicles that are operated in extremely wet environments (including off-road).
• The best advice is to always follow the vehicle manufacturer's recommendations as to which type of brake fluid to use. For most vehicles, that's going to be DOT 3 or DOT 4 brake fluids.

• There are a variety of ways to check the condition of the brake fluid. A simple visual inspection is not adequate because you can't tell the condition of the fluid by its color alone. Some new fluids are almost clear while others are yellow. A darkish appearance doesn't necessarily mean the fluid is bad, but if you see visible sediment in the master cylinder fluid reservoir, the fluid is obviously long overdue for a change.
• In one test, brake fluid samples were collected from 86 vehicles. The fluid ranged in color from light amber to dark murky brown. Testing revealed that the moisture content of the light amber fluid samples (which most technicians would probably say looked OK) contained as much moisture (1.1 to 4.5%) as many of the darker samples (which ranged from 0.9 to 7.0% moisture content). In fact, there were some darker-looking samples that actually contained less moisture than the lighter color samples. The conclusion? There's no correlation between the color of brake fluid and its water content.
• To determine the fluid's condition, you have to test it chemically, physically or electronically.
• One such method is to use chemical test strips that react to ions produced by the breakdown of corrosion-inhibiting chemicals in the fluid. These test strips are relatively inexpensive (less than 50 cents apiece), easy-to-use and can detect fluid breakdown in its earliest stages. The strips are color coded with a scale of 0 to 100 so you can judge for yourself the condition of the fluid.
• There are also chemical test strips that react to the presence of water in the fluid. This type of test strip changes color to indicate the degree of contamination. If the fluid contains two to three percent water, it is considered "borderline." Three percent or more water calls for a fluid change.