Several times a year, clients bring vehicles to us with major brake system problems. The symptoms include the brake pedal going to the floor or the wheels locking up. These vehicles had no previous problems and often do not have high mileage. An improper fluid is added to the brake system, and the result is very expensive.
Brake fluid is different from other fluids
Many automotive fluids are petroleum based. Automatic transmission fluid, engine oil and power steering fluid are examples of oil-based fluids. Engineers specify alcohol-based fluid for brakes and it is not compatible with petroleum fluids. They use alcohol because it resists heat and cold and helps absorb any moisture that enters the system.
They design the rubber used in the braking system for high pressure sealing. Manufacturers also design this type of rubber for use only with brake fluid. Severe damage results from even the smallest amount of petroleum-based fluid added to the brake system. Oil-based fluid causes the rubber in the braking system to swell and very rapidly deteriorate.
The most common mistake is adding power steering fluid to the brakes
Power steering fluid contamination will cause seals to immediately begin swelling. As the seals swell, they move forward and block the passages that allow the brake system to function. One example is the return ports in the brake master-cylinder. The swollen seal blocks this port and the return of fluid to the reservoir, when we release the brake pedal.
The heat produced by braking causes the brake fluid to expand. If the expanded fluid does not return to the reservoir, the brakes apply and lock the wheels. Common symptoms are the brakes locking up after driving a short distance. Almost immediately, the contaminants break the rubber down, and the system fails. A brake pedal that goes to the floor is often the next symptom. Complete brake system failure is also common.
Both power steering and brake fluids are clear to amber in color and the containers often resemble each other. People sometimes make this mistake on their own vehicles and other times well meaning quick-lubes may make it for them.
How petroleum damages the brake system
When a petroleum-based fluid, such as power steering fluid, enters the brake system, damage begins immediately. The first damage is usually to the reservoir cap and master cylinder seals. When we apply the brake pedal, the master cylinder quickly pumps the contaminant into the rest of the system. The only effective repair is disassembly of the system, cleaning the lines and replacing every rubber component. Petroleum soaks into rubber and any contaminant left in the system will cause the process to repeat.
Brake fluid is heavier than petroleum-based fluids and does not automatically mix with them. If we discover the error before we apply the brakes, we could syphon the contaminant from the master cylinder immediately. We then purge the system in reverse, forcing all fluid to flow back to the master cylinder. Replacing the contaminated master cylinder and bleeding the brakes could repair the system.
Such a repair is rarely possible as most often people apply the brakes before they discover the error. Hydraulic pressure has more than enough force to push the contaminant deep into the system and thoroughly mix it.
In the illustration above, we mix 40 ml of brake fluid with 40 ml of power steering fluid and agitate the fluids. After an hour they begin to separate, but 10 ml remain mixed. Even after 24 hours this mixture remains in tact. Once mixed the petroleum contaminates the rubber components of the system. Flushing this out is not possible and the oil will continue to contaminate any new components we install.
Proper repair can be quite expensive, especially on a modern vehicle with Anti-lock braking. Because of the costs, it may be tempting to replace only the master cylinder and perhaps the brake calipers. This results in repeat contamination as the petroleum soaked into other parts again enters the system with use. Failure to remove every contaminated component causes repeat failure.
Brake systems are sealed, when low, it is low for a reason
It is best to realize that low brake fluid suggests a problem. As the brake pads wear, the system draws in fluid to allow for the additional travel of the pistons. A low fluid level often shows worn brakes. Another cause for fluid to drop is a leak in the brake system. The reason for the low fluid should always be checked. Virtually, all systems have warning lights to warn of brake fluid level that has fallen.
Once opened, a bottle of brake fluid will begin absorbing moisture from the atmosphere. After breaking the seal on the bottle, we cannot store brake fluid, and unused fluid should be disposed of. We should always purchase a new, unopened bottle of brake fluid to replenish our system. Opened bottles of brake fluid become contaminated and will damage the system.
Most domestic and Asian vehicles use DOT 3 fluids. European vehicles often use DOT 4, which has a higher initial boiling point. Both DOT 3 and four are alcohol-based and we can substitute them in an emergency, but we must follow certain cautions until it is replaced. Check the brake fluid cap or owner’s manual for the proper type.
DOT 4 brake fluid contains more alcohol than DOT 3 and thus has a higher initial boil point. This also makes it more susceptible to moisture, and it must be replaced more often than DOT 3. Installing DOT 3 lowers the boiling point of the system and the temperature at which brakes may fail.
Being cautious, when topping off fluid levels is wise especially when having the engine oil replaced. Instruct quick-lubes to advise you if the fluid is low and not try to top it off. If a contaminant is added to the system, immediately towing the vehicle to a service facility may save a major expense. Do not allow anyone to apply the brakes. Reverse-purging of the system immediately may limit further damage.
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