In this article you'll learn why it is important to regularly change your old brake fluid with new brake fluid, how to bleed your brake system and how to choose the right fluid for your vehicle.
The important thing to remember is, brake fluid can absorb moisture, this is bad because of the 'wet boiling point' which is the reduced boiling temperature of fluid that's absorbed 3.7% water. It only takes about two years for DOT 3, 4, and 5.1 fluids to reach 3.7% water content, this is why an occasional brake fluid flush can be good preventative maintenance.
The engineers that built your vehicle specified replacement intervals for brake fluid because your vehicle requires it for a good reason. Replacing old brake fluid with fresh new brake fluid, is not just important - it's essential.
What is a Brake Fluid Flush?
A brake fluid flush or brake fluid change refers to the process of replacing all of your old brake fluid, in turn removing any moisture from the brake system with clean, fresh brake fluid to allow the brake system to perform at its optimum temperature without boiling.
This involves pushing the old fluid out of the entire system as new fluid is added, creating the pressure necessary to bleed out the old fluid.
A brake fluid flush ensures proper brake system performance and increases the lifespan of your braking system and brake components.
Why are Brake Fluid Flushes Important?
As brake fluid naturally absorbs water over time, it becomes contaminated and boils sooner at lower temperatures. This can significantly reduce brake effectiveness - especially if antilock-braking-systems (ABS) are involved.
Most brake fluid used in production vehicles today is glycol-based. This type of fluid is 'hygroscopic', which means it naturally absorbs moisture that's present in the air at all times. According to engineers, that absorption rate is approximately 1.5 to 3% a year in areas of normal atmospheric pressures. In humid climates, that rate can climb even higher.
Moisture will always find its way into the lines through microscopic pores in brake hoses, seams, joints, and seals - there's simply no way to avoid it.
As brake fluid absorbs more and more water, it will begin to boil at lower and lower temperatures. Remember that brakes work by converting energy - forward motion - into friction, which is dissipated as heat. The heat in the pads and rotors eventually makes its way to the brake fluid in the brake hoses and lines.
In normal liquid form, brake fluid does not compress - this allows pressure in the system to be transferred consistently and evenly. Boiling is bad because when fluid changes into a gaseous state and aerates into bubbles, it does compress - creating a squishy-feeling pedal and reduced stopping power. In a worse-case scenario, the brake pedal may sink to the floor, giving you no braking at all. Performing a brake fluid flush prevents this gradual decay in the effectiveness of your braking system.
While water in the brake system is the main reason fluid flushes are performed, the hygroscopic nature of brake fluid actually proves helpful in preventing other types of problems.
For example, if brake fluid was not hygroscopic, water (which is heavier by weight) would pool together in pockets and gravitate to lower areas in the system such as the brake calipers. This would create a more serious corrosion problem, and those pockets of water would boil much sooner than when water is dispersed evenly throughout all of the fluid.
It's also important to note that corrosion of metal brake lines and moving parts such as calipers and master cylinder pistons can and will occur eventually if moisture in old brake fluid builds to a significant point. These metal parts can actually corrode from the inside out. A brake line which fails can lead to partial or complete brake failure.
Brake fluid testers such as this one sense and report the percentage of moisture in brake fluid.
Gritty corrosion on parts surfaces causes seals to wear and leak, and it causes calipers to bind up to the point where they no longer apply or release the brakes effectively.
Preventing corrosion of expensive-to-replace brake system components is another important reason you should perform brake fluid flushes at the intervals recommended by your vehicle manufacturer.
A brake fluid tester is a useful tool for monitoring the condition of your brake fluid, because it senses and reports the percentage of moisture that's been absorbed.
How to bleed a brake system?
Bleeding your vehicles brake system can be done in several ways. You can either have another person pump the brake pedal manually or by using a hand-pump pressure bleeder tool, alternatively by gravity bleeding the brake system.
Old brake fluid is removed by loosening and tightening bleed nipples. Bleed nipples are found on brake calipers and or drum brake wheel cylinders, AND not forgetting the bleed nipple on the rear brake load compensator valve above the rear axle if fitted, this is very often forgotten about as not all vehicles have them fitted, however most Toyota 4x4's including the Hilux Pickup, Hilux Surf, 4Runner & Land Cruisers do have them fitted. These valves are often found seized and for best results will need to be replaced.
When bleed nipples are loosened with a bleed nipple spanner, fluid can be bled out directly through them in stages. It is always best to start bleeding a system from the furthest point away from the brake master cylinder. For example if the brake master cylinder is on the drivers side (right hand drive), then you must start bleeding from the passenger side rear, then the drivers side rear, followed by the passenger side front and finally the drivers side front, not forgetting to bleed the brake compensator valve.
Manual Bleeding (Pedal Pumping)
Grab yourself a brake bleeding pipe, brake nipple spanner and preferably a clear bottle or jar topped upped with about an inch or two of brake fluid. Manual bleeding requires a second person to manually step on the brake pedal at designated intervals, whilst the other is loosening and tightening the bleed nipples. Remember to keep the brake pedal pushed down when tightening the bleed nipples after all the air locks have been removed, you will know this when the fluid is able to flow freely without any bubbles forming in the excess brake fluid bottle.
Bleed nipples should be opened one at a time, starting at the wheel farthest from the master cylinder. After the pedal is fully depressed and brake fluid has flowed out through a tube that's submerged in a container of old fluid, the bleed nipple should then be closed so that air does not get sucked back into the lines when the pedal rises back up. As this process is repeated at all four wheels, fresh brake fluid should be added to the reservoir so that new fluid continues to come out of the lines.
Follow the step-by-step below;
- Fill up the brake master cylinder or reservoirs with brake fluid. Check the reservoir after bleeding each corner, add fluid if necessary.
- Connect a vinyl or silicone bleeding pipe to the bleed nipple on the brake caliper, wheel cylinder or compensator valve. Insert the other end of the tube into about an inch or two of brake fluid in a clear bottle or jar.
- Crack open the bleed nipple & begin bleeding the brake lines starting with the longest hydraulic brake pipe. Slowly pump the pedal until fluid begins to flow, close the bleed nipple holding the pedal down when the air & bubbles have been removed.
- Repeat the procedure for each bleed nipple on the entire brake system. Remember to bleed the brake compensator valve above the rear axle.
Pressure bleeding is a process for purging old brake fluid and air out of the system by using a canister with a built-in hand pump. Using a hand-pump pressure bleeding tool, whether power or manual, fits over the brake fluid reservoir and is secured in place with the use of adapter pieces. As pressure and new fluid are added, old brake fluid is removed by loosening and tightening bleed nipples. Bleed nipples are found on brake calipers and or drum brake wheel cylinders. When the bleed nipple is loosened with a bleed nipple spanner, fluid can be bled out directly through it.
Because pressure is created and held by the pressure bleeder unit, one person is able to do this job easily.
Gravity bleeding allows the fluid & air to flow out on its own pushing out unwanted air in the brake system. Gravity bleeding is one man job, simply remove the brake master cylinder cap to create a vacuum and crack off a bleed nipple to allow the fluid to flow freely pushing out any air in the system.
This is how RoughTrax used to bleed our own customers vehicles when we previously carried out mechanical work.
What Type Of Brake Fluid Should I Use?
The different grades of brake fluids determine the differentiation of the dry and wet boiling points. Note that DOT2 & DOT3 brake fluid is obsolete and largely no longer available as they are now superseded by DOT4 Brake Fluid as a recommendation for a higher boiling point.
Different grades of brake fluid have different dry and wet boiling points. A 'Dry boiling point' signifies the boiling temperature of brand new, pure fluid with no moisture in it.
The 'Wet boiling point' is the reduced boiling temperature of fluid that's absorbed 3.7% water. Because it only takes about two years for DOT 3, 4, and 5.1 fluids to reach 3.7% water content, doing a brake fluid flush every several years is extremely important.
The Department of Transportation (DOT) assigns the brake classifications. Each DOT designation signifies fluids have been certified to meet performance standards up to designated temperature points which are:
DOT3 (Glycol-based) Brake Fluid
Commonly specified by auto manufacturers, this type of brake fluid meets criteria for glycol-based fluids with a wet boiling point of 284 degrees Fahrenheit and a dry boiling point of 401 degrees F. It has not been superseded by DOT4
DOT4 (Glycol-based) Brake Fluid
Most vehicle manufacturers specify using only DOT3 or DOT4 brake fluid, RoughTrax supply and recommend DOT4 brake fluid for all brake and clutch applications.
Like DOT 3 fluid, DOT 4 is also glycol-based. It's suited for heavier-duty use and higher brake temperatures thanks to a higher wet boiling point of 311 degrees Fahrenheit and a dry boiling point of 446 degrees. Vehicle manufacturers of higher-performance models that see aggressive use on the street and/or racetrack often use DOT4.
DOT5 Synthetic (Silicone-based) Brake Fluid
This is a recently-developed silicone-based synthetic brake fluid that has a boiling point of at least 500 degrees Fahrenheit. Because it does not absorb moisture like DOT 3 & 4 fluids, DOT 5 fluid doesn't need to be changed as a matter of routine maintenance. And like synthetic motor oil, DOT 5 fluid can cost three to five times as much.
DOT 5 is NOT suitable for ABS systems because it can aerate when cycled through small orifices. DOT5 can be found in use on military trucks and heavy equipment not equipped with ABS that sit for long periods of time. Additionally, DOT5 will not mix with types 2, 3 or 4 fluid.
According to engineers that work with brake fluids, silicone in DOT5 that's introduced into an older brake system can attach itself to sludge that may already be present due to un-related component deterioration. This can create gelatinous glop that grows bigger as it draws other contaminants to it. Eventually, metering orifices become clogged and pistons can even stick. Engineers also say that if you've already changed over from 3 or 4 to DOT5, you should switch back. If you've ever gotten silicone on your fingers, you understand how difficult it is to wipe away fully.
DOT5.1 (Glycol-based) Brake Fluid
This is the non-silicone version of DOT5. Both have the same boiling points, but 5.1 embodies all the other physical characteristics of DOT 3 and 4.
Since DOT 4 and 5.1 are both glycol-based brake fluids they are compatible with each other, which means they can be readily mixed without harming your brake system. It is important never to mistake DOT 5.1 (glycol-based) with DOT 5 which is silicone-based and should never be mixed with any other DOT fluid.