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Is it time for a fluid change?

Hydraulic fluid is used in your vehicle

in a number of different ways, typically:-

  1. Your brakes

  2. Your clutch

  3. For some vehicles your steering

However, a fluid change at certain service intervals is often forgot about and when we're

thinking about our brakes, is ever changing the fluid really necessary?

DOT brake fluids are hygroscopic – they tend to absorb water from the environment, and this absorption cannot be prevented. On average, the brake fluid in a vehicle absorbs up to 1.5 % of water each year. As the water content of the brake fluid increases, its boiling point decreases. When this happens, a failure known as “vapor lock” occurs and the force applied to the pedal is not transmitted to the brake cylinders, potentially leading to a failure of the brake system. Therefore, the boiling point and the water content of brake fluids are essential parameters when considering the safety of a vehicle.

Many car manufacturers call for changing of the brake fluid at 24 to 36 months.

In fact, according to a report by the National Highway Traffic Safety Administration in America, 20 percent of the cars it tested had brake fluid with 5 percent moisture content. 3 percent moisture content in DOT 3 brake fluid reduces the boiling point of the fluid by more than 100 degrees.

As mentioned, water/moisture in the brake fluid lowers the boiling point which is undesirable because as the brakes get hot during use, some of the heat is transferred into the brake caliper and thus into the hydraulic fluid. During rigorous braking temperatures can become so high that the brake fluid boils, introducing bubbles of gas into the hydraulics. This is very bad news since the bubbles of gas increases the overall compressibility of the brake fluid leading to a spongey pedal/lever and in extreme cases can lead to complete brake failure. Many track day drivers will recall a hair-raising moment when their brake pedal or lever bottomed out completely after boiling their brake fluid.

Brake fluid is one of those things that can escape us. When a car needs a service there are things we think of straight away: oil, oil filter, air filter, spark plugs.

But how often do you think about your brake fluid? Exactly. And we at Haynes don’t want you to come a cropper.

As such, we’ve put this guide together for you, so you know when and how to change your brake fluid.

How brake fluid works Unless you drive something really, really old, your car is going to have hydraulic brakes. And that’s good, because hydraulic braking systems are commonplace and also the most effective. However, they’re only any good if they’re maintained properly. Yes, there is the matter of discs and pads, but don’t go forgetting the fluid. Without this, your brakes won’t do anything. When you press the pedal, you act upon the fluid, which then in turn acts upon the brakes. The problem with brake fluid is that it’s hygroscopic. That means it likes to suck up moisture. The more moisture that becomes a part of the brake fluid, the less effective the fluid is. So, logic would dictate that the longer you leave it, the less effective it will become.

Can brake fluid go bad in any other way? If there is a fault with your braking system at any point, and the fluid gets too hot, you can boil it. This will have a lasting effect on the fluid’s viscosity and as such, its effectiveness. In fact this can also happen when the car is driven enthusiastically (on a race track, for example), where the brake fluid can overheat and cause something called brake fade, where the hydraulic action of the fluid is less effective. Take the cap of the brake master cylinder and smell the fluid, if it smells burned, or if it’s exceptionally dark in colour, it’s time to change it. While some fluids are technically interchangeable, it’s always best to make sure you’re filling your braking system with the right stuff.

How often should I change my brake fluid? Well, this is a tricky one to answer, because it varies from car to car. The general rule is that brake fluid should be good for four to five years. However, there are other factors that can play into its lifespan. If you have a heavier car you'll need to knock a year or two off that, while a lighter car might get a bit more life out of its fluid. If you're very diligent about the care of your car, look to change your brake fluid every two years at the minimum, though it won’t hurt to do it once a year if you’re doing a lot of miles. Even if the fluid looks okay, you should still change it. You’re not going to see the moisture in the fluid – it happens on a microscopic level. The best and most definitive way to know is to check your trusty Haynes Manual. One thing to consider about brake fluid is that even if you’re not changing it, you should regularly check it. As a bare minimum, you should be giving it a cursory inspection once a year. Which leads us neatly to…

How can I tell if my brake fluid needs to be replaced? Brake fluid is carefully engineered to have a certain thickness, hence the 'DOT' rating you'll see on the bottle when you buy it. Your brake fluid should always be clear. If it has any dirt or murkiness, it needs to go. If it smells burned, it also needs to go. Other indicators will be if the level drops on the reservoir (it goes down as the brake pads wear, so don't automatically think that there's a leak), or if you notice that the brake pedal is spongier than it used to be when pressed.

When carrying out a service we automatically check the moisture content of all of the fluids. However, if you would like us to run a moisture check on your vehicle, we can do so for no charge.

Below is a I.M.I. report published this month:- Author: Hayley Pells FIMI Published: 01 Nov 2021 Updated: 01 Nov 2021

Boiling point for brake fluid

Brake fluid plays a critical function within a hydraulic system, but can often become overlooked and has become a more complex operation. Due to the hygroscopic (it absorbs moisture from the air) nature of commonly used glycol-based brake fluid, accurately measuring water content is a critical part of routine maintenance. And since 2018, contamination of the fluid has been part of the MOT inspection, so it’s worth knowing your DOT 3 from your DOT 5 fluids.

Why water is a problem? Manufacturer’s data generally gives a 24-month lifespan for common use brake fluid, factors such as the age of the vehicle, changes in ownership, interruptions like a pandemic – can all contribute to the potential of missing this important routine change. High-water content accelerates corrosion in the system, the expansion caused by ice can compromise integrity, and when the brakes transfer heat to the fluid the water can vaporize in the system creating gaseous pockets, rendering the hydraulic system inoperable.

Used during the MOT, brake fluid is checked for level and contamination. However, removal of the cap is not permitted and aged vehicles may make this method of inspection difficult. Brake fluid that has turned to the colour of cola is an easy contamination to spot, but it does mean only the very worst presentations are issued with a reason for refusal. A visual inspection during routine maintenance doesn’t have this restriction, but if you are going to take the cap off testing the fluid is advisable as normal, straw coloured brake fluid, may still have a high-water content. Get the measurements right There are two common methods to measure the water content in brake fluid. One is using a dip style pen device that utilises a traffic light LED display to indicate the approximate water content. This is an economic solution and doesn’t remove any fluid from the reservoir. The second method takes a sample of the brake fluid and boils it, allowing that boiling point to be very accurately recorded. The equipment used for this can be multiple times more expensive and requires replacement fluid if the sample proves it’s still useable. The price point of the equipment makes this a more unusual choice, but the high degree of accuracy could make the investment worthwhile. Fluid exchange Whenever the brake hydraulic system is opened, it introduces air into the system. Historically, changing the brake fluid was a manual process, using an assistant to press the brake pedal while each line was bled. This can still be a common practice in older vehicles but most modern vehicles require a diagnostic tool used with a pressure brake bleeder to change the fluid. This has the advantage of becoming a one-person operation and it’s essential for modern systems as the use of pressure and the diagnostic tool removes not only the air from the system, but any remaining debris. Left unchecked this can cause component failure in items such as the ABS module. Although this can be a faster operation, it can often be more expensive owing to the greater tooling investment, the maintenance of that equipment and the need to continuously upgrade the database for new vehicles. Referencing manufacturer’s information is essential, both for the selection of the correct brake fluid (different types should not be mixed) and also for the prescribed technique for change. And although it’s possible to use a one-person operation using a pressure bleeder for older brake systems, it isn’t recommended for a manual operation for a brake system that requires diagnostic equipment and/or a pressure bleeder. And in the age of electrification, it’s worth remembering that some high-voltage vehicles may use regenerative braking meaning there could well be two hydraulic systems to bleed.

Common brake fluids and their wet boiling points DOT 3 glycol based 140°C

DOT 4 glycol based (most common) 155°C

Super DOT 4 glycol based 195°C

DOT 5.1 glycol based 185°C

DOT 5 silicon based 185°C

Symptoms of air in the brake system Spongey brake pedal


Methods of inspection

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