Oil VII - The Power To Stop
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Words Freddi Stafford Words Freddi Stafford

In this, our seventh and concluding article on how automotive lubricant technology has changed forever, we will discuss brake fluid − which is not something you can use until you notice deterioration.

With old fluid, one minute you have perfect brakes; and the next you have none.

Many people are complacent about their brake fluid, assuming that it will always do the job. It is a "safety critical item", but how many motorists ever ask to have their brake fluid checked, or when it was last changed?

Approximately 70% of vehicles on the road have brake fluid that will boil at 175ºC or lower, severely reducing the efficiency of brakes; and I am hoping that this is because no-one has ever informed the drivers of the importance of brake fluid.

At least 30% of motorists have brake fluid in their system that is so poor that they risk their own lives and those of fellow motorists every time they drive.

Why is brake fluid important?

It may not cross your mind every time you stop your car, but it is brake fluid that gets the job done for you.

Your car’s brakes are its most important safety feature, and brake fluid (although one of the most neglected fluids in vehicles today) is vitally important for safe driving. In fact, your old, water containing glycol brake fluid is downright dangerous and cannot be relied upon to stop your car reliably.

What effect does water have on brake fluid?

Even in normal operation, considerable heat is created during braking, and this heat must be dissipated fairly quickly for effective operation. Brake fluid must therefore have a high boiling point to remain effective.

However, brake fluid is hygroscopic, in that it absorbs water naturally from the atmosphere (from the humidity in the air) even if the car isn't in use.

Most water comes from the vent in the master cylinder cap and the resultant condensation in the air space above the fluid.

Water absorption also takes place through the flexible rubber hoses, where gases pass through, but fluid cannot leak out.

Removing the reservoir cap to check the brake-fluid level also increases the exposure of the brake fluid to moisture-rich air. Over time, the water contaminates the brake fluid, lowering its boiling point.

Under heavy braking, such as a long downhill descent or towing, the brakes become hot and heat up the brake fluid.

Water boils at 100°C, evaporating into steam, which (unlike fluid) is compressible.Although you can't compress a liquid (like brake fluid), you can compress a vapour like steam.

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