Does brake fluid need changing

Does Brake Fluid need to be changed?

What is brake fluid made of and why does it have to be changed?

Brake fluid is a type of hydraulic fluid used in Anti lock brake systems and clutch systems in cars, light trucks and motorcycles. It is used to create force which creates pressure in the brake lines and transfer that pressure to the brake pads.  It works because liquids do not compress under pressure and their molecules pack together like a snowball.  So pressure applied to the brake pedal moves the fluid and transfers pressure to the pads. 

Because oils damage rubber seals and hoses brake fluids are not oil based. Most brake fluids used today are glycol, mineral oil or and silicone (DOT 5).  

For reliable, consistent brake system operation, brake fluid must maintain a constant thickness under a wide range of temperatures (including extreme cold). Brakes fluids must not corrode the metals but also must protect against corrosion as moisture enters the system. Most fluids have additives that are added to the base fluid to help with this. Brake fluids must maintain a low level of compressibility that remains low, even with varying temperatures.

So, why does my mechanic say it has to be changed? Brake fluid is not like a set of tires bought at Big-O!

This may come as a shock to most people, but 75% of car owners never change their fluid. Brake fluid attracts moisture, and moisture can rust the insides of the brake system. This moisture was not that big of deal when 10-15 years ago but brake systems are very different than they were back then and even a small amount of rust on delicate ABS systems can do a lot of damage.  Brake fluid can also break down over time from excess heat that is created from within and begin to compress when put under pressure.

Many shops recommend flushing the brake fluid system about every 30,000 miles, or whenever they are performing a brake job. To flush, all the old fluid is allowed to drip out.  Once the fluid is taken out, new fluid is put in.  As a consumer, make sure the action taken by the mechanic is a flush not a bleed.  A flush is getting all the old fluid out, a bleed is just putting in enough new to fill up the master cylinder and the lines. Kind of like a patch on an old wineskin.  

How can you tell if your fluid has water in it? There are test strips which can detect high levels of moisture present in the brake fluid, which should be almost clear. Auto stores carry these strips. If the fluid is black, don’t even bother checking with strips because the fluid is contaminated with dirty water and needs to be changed ASAP.  Again, if your fluid is not yellowish or clear, it needs to be changed.   


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