Ritchie owns a vehicle repair garage and is a qualified MOT tester, based in the UK.
Brake Fluid Absorbs Water (It's Hygroscopic)
Glycol-ether (DOT 3, 4, and 5.1) brake fluid is hygroscopic, meaning that it absorbs moisture from the air over time. This moisture can affect a braking system in a number of ways.
As more water enters the brake fluid, the boiling point is reduced. This is a problem as the water can vapourise in the braking system, causing an air void, resulting in a 'soft' or 'spongy' brake pedal and reduced braking efficiency.
In extreme cases, braking distances increase, making a vehicle difficult to stop or not stop at all.
Brake fluid is often referred to as 'dry' (no water has been absorbed) or 'wet' (containing water).
How Often Should Brake Fluid Be Replaced?
Most vehicle manufacturers recommend that brake fluid be replaced every 12 to 36 months in normal operating conditions. The exact timescale depends on how humid the conditions in your area are.
I recommend to my customers that they have their vehicle's brake fluid replaced every 24 months.
You should consult your vehicle handbook or dealership for the exact recommendation for your vehicle and location.
Old Brake Fluid Can Damage Brake Components
Water in brake fluid can have a negative effect on braking components.
Moisture in brake fluid can encourage corrosion, resulting in sticking or seized callipers. A seized brake caliper can result in reduced braking efficiency or cause a vehicle to 'pull' in an adverse direction when applied.
Seized brake calipers can result in brakes sticking or 'snatching', causing brake pads to wear excessivly early or unevenly.
Water in brake fluid can also cause rubber on wheel cylinders to perish, resulting in leaks which may contaminate brake shoes within the drum.
Corrosion of brake pipes can occur, particular in copper brake pipes, and may cause flexible brake hoses to bulge or split over time.
What Is Brake Fluid Made From?
The majority of vehicles use Glycol-ether (DOT 3, 4, and 5.1) brake fluid. Glycol-ether is a solvent which has a high boiling point and low level of compressibility.
Some other forms of brake fluid include mineral-based and silicone, but these are less popular than glycol ether.
What About DOT 5 (Silicone) Brake Fluid?
DOT 5 (silicone) brake fluid is sometimes used in a vehicle, mainly in very cold climates. However, it must not be mixed with other non-silicone brake fluids and can only be used in a vehicle where total brake fluid replacement is to occur, such as a restoration or total brake system replacement.
DOT 5 brake fluid is hydrophobic, which means that it doesn't absorb water. However, it can result in a spongy brake pedal as it reaches a higher boiling point and is generally not used in road cars.
What Is LHS or LHM Mineral Based Brake Fluid?
LHS and LHM (Liquide Hydraulique Minéral) brake fluids were designed specifically for Citroen vehicles, which had hydropneumatic suspension. Whilst the LHM fluid absorbs less moisture than DOT 3, 4 and 5.1 fluids, it can reach boiling point more quickly and therefore is generally not used in the majority of cars.
How Is Brake Fluid Replaced?
For most vehicles, the process for replacing (flushing brake fluid) involves removing brake fluid from the braking system by pushing new fluid into the system to remove the old fluid.
This is usually achieved by opening bleed nipples on the further brake line away from the brake fluid reservoir and pumping (or pulling) the brake fluid through the pipes until all the old brake fluid has been replaced with new.
Professionals may use a brake bleeding machine to create a vacuum to pull the old fluid through the system. The machine adds new fluid as the old fluid is evacuated from the system. In other examples, an assistant can pump the brake pedal to force the old fluid through, topping up the reservoir as the level drops. The following video from Youtuber ChrisFix explains the process in more detail:
Can Brake Fluid Be Tested?
Brake fluid can be tested for water absorption using a specialist testing tool. I recommend the Laser brake fluid tester. It's a quick and reliable way to test if brake fluid is 'wet' or 'dry'.
How Much Should a Brake Fluid Change Cost?
According to the website autoservicecosts.com, a brake fluid change costs between $73 and $104 for the majority of vehicles in the USA. In the UK, expect to pay between £30 and £100.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.