We do quite a lot of vehicle inspections.

Sometimes, for a prospective owner, before purchase and sometimes for regular customers who have come to realise that
a bi-annual, six monthly check up keeps  them on the road and cuts off serious problems before they arise.

The results of such inspections can be broken down into two categories.

Safety and reliability.

Reliability items, if not repaired, may stop you from starting in the middle of winter, or will illuminate the check engine light,
or may, very occasionally, cause the car to stall, sometimes without being able to restart. But in most cases, the reliability
issue is not life threatening.

Embarrassing and expensive, yes, but not life threatening. 

Safety items, however CAN be life threatening, because if your steering, brakes, or suspension let go, it's hard to predict just
where you might end up - against a tree or in a hospital bed. And of all the safety items we highlight in our inspection reports,
the one that gets the least reaction from clients is "brake oil contamination".

This is understandable, because many auto manufacturers don't have any recommendations for changing brake fluid.
Check the level and if it is good, then all is okay. However, changing your brake fluid can protect your braking system from
expensive corrosion.

Most auto manufacturers use DOT (Department of Transport) type 3 brake fluid.
A few use DOT 4 type fluid, which has a higher boiling point.

These fluids are alcohol and as such are hygroscopic. Which means they will absorb water.
A container left open  will absorb water from the air. In the brake system, it will slowly absorb humidity from the air.

DOT 5.0 or 5.1 type are silicone based. This fluid is blue in colour so it is obviously different from other brake fluids.
The apparent advantages of silicone brake fluid are that it does not attract and hold water, but it can leak more easily.
Most auto manufacturers use DOT 3 fluid and the two types of fluid should never be mixed.
If you want to use silicone-based brake fluid, the entire system should be purged of the alcohol-based fluid.

So when should you change or flush brake fluid?

First, the colour of the fluid is no indicator of fluid quality. Discoloured brake fluid can still be good and light amber fluid
can be quite poor.

Because brake fluid is hygroscopic, water in the brake fluid definitely lowers the fluid boiling point so that in heavy use,
it may boil and cause brake fade, which shows up as long pedal travel and soft brakes.
Water will also cause corrosion problems, however, research has found that newer vehicles do not absorb much water
into the brake system. Transparent plastic fluid reservoirs don't need to be opened to the air and improved materials
seal moisture out better. So if it isn't water that causes problems, what can it be?

Copper is the modern answer.

Just like anti-freeze, corrosion inhibitors in the brake fluid reduce the breakdown of materials in the brake system.
As these inhibitors wear out due to age, water content and high heat, generated when the brakes are used, copper in the
brake line material starts to corrode. When enough copper is present in the brake fluid, it  acts as an oxidizer and starts to
corrode other parts such as calipers, wheel cylinders and master cylinders.

The amount of dissolved copper in the brake fluid is a good indicator of brake fluid quality.
But most service garages, including mine, cannot test the fluid so we don't really know if the brake fluid quality is good or not.

As a medium compromise that puts expense up against expediency, a brake flush every two years is a small price to pay.

Whenever we have to change a calliper or other brake component that requires a removal of air entrained in the fluid,
we always try to keep pumping the pedal until we're sure that only new fluid remains in the system.