Whether drum or disk brakes the force of stopping is transferred by a liquid brake fluid. Like most automotive fluids, brake fluid has a service life and needs to be replaced. What is confusing is that even experts do not agree on when.
Types of brake fluid and boil points
Modern vehicles use hydraulic brake fluid derived from alcohol. Alcohol is chosen partially due to its ability to deal with temperature extremes. Brake fluid does not freeze until around -40 degrees Fahrenheit. When it is dry, brake fluid can withstand well over four hundred degrees Fahrenheit without boiling.
This is important due to the high temperature at which brakes operate. The Department of Transportation (DOT) rates brake fluid as to temperature resistance. DOT 3 has a minimum boil point of 401 degrees Fahrenheit when dry. DOT 4 is rated at 446 degree Fahrenheit, again when dry.
Alcohol is hygroscopic, it absorbs moisture from its surroundings. This is good, as any moisture in the system is absorbed. Unfortunately moisture also significantly lowers the boiling point of the brake fluid. For instance DOT 3 with 3.7% moisture boils at 284 degrees and DOT 4 at 311 degree Fahrenheit.
DOT 3 is largely used in domestic and many Asian vehicles. European and some Asian vehicles specify DOT 4. DOT 4 contains more alcohol and has a higher initial boil point than DOT 3 but absorbs moisture quicker and must be replaced more often.
Silicon brake fluid such as DOT 5 is sometimes used in classic cars and recreational vehicles. Silicon based fluid is less likely to corrode the system and does not damage paint like alcohol fluids. They also have a tendency to foam when agitated and cannot be used with anti-lock braking systems.
Moisture in brake fluid shows almost no symptoms
Another problem with moisture in brake fluid is that it causes corrosion. Moisture laden brake fluid begins to attack the metal in the brake system. Experts disagree on precisely how long this takes. Most agree between 2 and 3.5% moisture per year may be absorbed.
Much of the modern brake system is comprised of aluminum, a highly reactive metal. Oxygen in the absorbed moisture combines with aluminum to form aluminum oxide. This damages the metal components. Once the surface finish of hydraulic cylinders are corroded, they begin to leak and must be replaced.
Replacing hydraulic wheel cylinders and brake calipers is expensive. Replacing brake master cylinders and anti-lock brake system (ABS) control units are far more expensive and complex. Often the first symptom of a contaminated brake system is an ABS and/or red brake warning light and a hefty repair bill.
Brake fluid in a sealed system or container has a service life of around two to five years. In humid areas or when the system is opened contamination occurs more quickly.
The color of brake fluid alone is a poor indicator of condition. Contaminated fluid can appear clean and some fluid darkens quickly even though still okay. Testing brake fluid for contamination requires a refractometer or test strips made for the purpose. In the absence of proper testing equipment replacing brake fluid every two to three years is a fair guide.
The brake system is sealed, low fluid is the symptom of another problem
Most modern vehicles use translucent reservoirs so fluid level can be checked without opening. The system should not be opened unless absolutely necessary.
Low brake fluid level can result from brake pad and shoe wear. As material is worn away during braking, the pistons in the calipers and cylinders extend. This draws more fluid into the system and out of the reservoir. Naturally any leak in the system will also cause the fluid level to drop. Most systems have a red warning light to signal a drop in the brake fluid level.
Simply adding brake fluid to a system without first inspecting for wear or leakage is irresponsible
Brake systems are sealed and do not need routine addition of fluid. When fluid is lost there is wear or leakage. Rather than adding fluid, the system should be checked and the source of the loss of fluid corrected.
Another major source of contamination is replacement brake fluid. Once the bottle of fluid is opened, air and moisture enter. Adding brake fluid from a container that has been opened may contaminate the brake system.
Very often the source of brake fluid loss may be leakage. When leakage occurs it is important to understand, the leak is a symptom and not the only problem. Simply replacing the leaking component will all but guarantee repeat failure and failure of other components. Most often, moisture contaminated brake fluid is the actual cause.
Solving and preventing brake fluid contamination
Proper brake service will include replacement of the brake fluid. When servicing a moisture contaminated brake system, the fluid should be completely purged before any component is replaced. If not, the contaminated fluid will quickly spoil the new part(s). Various methods of replacing fluid need to be used depending on the type of braking system.
Older systems can often be purged by adding clean fluid at the master cylinder reservoir and using the brake pedal to push it through the system. On more modern systems this may not work and may result in lost of brake pressure.
Modern systems use various bleeding patterns, active braking systems and control units. Attempting to bleed these systems without the proper equipment and methods will cause the pedal to go to the floor and not come back. This will usually involve towing the vehicle to a qualified shop and possibly involved repair. Without a thorough knowledge of the system it is best not to attempt hydraulic brake service.
Systems can also be extensively damaged by adding anything other than brake fluid. For instance accidentally adding petroleum based fluid, like power steering fluid, can quickly cause thousands of dollars in damage. Petroleum based fluids will cause the rubber used in the brake system to swell and fail. This cannot be flushed out and all rubber components affected will have to be replaced.
Attempting to flush the wrong fluid from a brake system will only spread the contaminate. If anything other than brake fluid gets into a brake system, never press the brake pedal or use the brakes. Instead draw out as much of the contaminant from the reservoir as possible and have the vehicle towed to a professional.
With proper service many brake fluid related problems can be prevented. Replacing brake fluid before it becomes a problem also saves money. Testing brake fluid for contamination is part of the AGCO General Inspection. AGCO, it’s the place to go.