Brake Fluid

Overview

Brake fluid is a hydraulic fluid that is used to apply the brakes on the car. The hydraulic pressure is created in the master cylinder by force from the driver’s foot and with assistance from the power brake unit. The pressure is then applied to wheel cylinders (for drum brakes) and brake calipers (for disk brakes) to stop the car.

Brake Fluid Types

Glycol Based

The most common brake fluid in use today is glycol-based. All the glycol-based brake fluids (DOT 3, 4, 4+, 5.1, and 5.1 ESP) share the same characteristics except each new release ups the boiling point, making the fluids forward compatible.

Glycol based brake fluids are extremely hygroscopic, meaning they will readily mix with water. They will even pull moisture out of the atmosphere. As the moisture content rises in brake fluid, the boiling point starts to drop. Note, from the table below, that the boiling point of DOT 4 brake fluid drops from 4460 F to 3110 F with just the absorption of 3.7% water.

Significant moisture content in brake fluid can cause a range of problems, such as:

  • The moisture can cause corrosion on the inside of the brake lines and other metal parts.
  • Higher levels of moisture can lower the boiling point of the brake fluid to the point that the fluid will vaporize during braking. This will cause almost immediate brake failure.

Glycol Based Brake Fluid Can Damage Paint

If you spill glycol-based brake fluid on a painted service, wash it off as soon as possible. If enough brake fluid sits on a painted surface long enough it can strip the clear coat or even bubble up the base paint. This often happens when a rag is used to wipe up some brake fluid and the rag is carelessly laid back on the car.

Mineral Oil Based

Liquid hydraulic mineral (LHM) is a brake fluid that evolved from Citroen’s hydropneumatic suspension system. It is made from mineral oil and is hydrophobic (the opposite of hygroscopic); therefore, it does not absorb water as traditional brake fluid does.

Mineral oil based brake fluid is not compatible with traditional brake fluid in any way. If you mix these two brake fluids up in the wrong system you must immediately do a full system flush or risk having to change all hydraulic components. To help identify a mineral oil based brake system the brake fluid is dyed green and the master cylinder cap is typically green also.

Mineral oil based brake fluid is used on some European cars, motorcycles, and bicycles.

Silicon Based

Silicone based brake fluid (DOT 5) has a high boiling point and is not hygroscopic (will not absorb moisture). However, it is not compatible with anti-lock brake systems. During an anti-lock event, when the solenoids are active in the ABS unit, the fluid tends to foam up.

Silicon based brake fluid cannot be mixed with traditional brake fluid. If a system is bring switched over from traditional brake fluid to silicone brake fluid all the old fluid must be flushed out.

Silicone Brake Fluid Will Not Damage Paint

Since silicon brake fluid will not damage paint and is popular in the following applications:

  • Show Cars – Show cars tend to have expensive paint jobs, even on the bottom of the car, where it would be damaged by a glycol-based brake fluid leak.
  • Motorcycles – Motorcycles tend to have expensive paint jobs that can be damaged by glycol-based brake fluid leaks.

YouTube: Types of Brake Fluid Discussion With Raybestos Brakes (6:55)

Brake Fluid Specifications Table

The boiling point data listed in the table below is the minimum as defined by DOT standards and most manufacturers produce brake fluids that exceed the minimum standards. This information is readily available from each manufacturer.

TypeDry Boiling PointWet Boiling Point1Base
DOT 34010 F2840 FGlycol
DOT 44460 F3110 FGlycol
DOT 4+24460 F3110 FGlycol
DOT 55000 F3560 FSilicone
LHM+34800 F4800 FMineral Oil
DOT 5.15000 F3560 FGlycol
DOT 5.1 ESP45000 F3560 FGlycol
  1. Wet Boiling Point – Defined as 3.7% water by volume. Brake fluid absorbs moisture from the air at a rate of about 2% per year.
  2. DOT 4+ – Developed with low viscosity to be compatible with some anti-lock braking systems (ABS).
  3. LHM+ – Liquid Hydraulic Mineral (LHM) is mineral oil and is not hygroscopic; therefore, water-vapor bubbles do not form in the system.
  4. DOT 5.1 ESP – Developed with low viscosity to be compatible with some anti-lock braking systems (ABS)

Brake Fluid Checks / Tests

Brake Fluid Level Check

“Max” and “Min” Lines on the Reservoir

Most master cylinders have a plastic reservoir marked with the brake fluid maximum “Max” and minimum “Min” levels. With a new set of brake pads, the brake fluid level should be at the “Max” line on the master cylinder reservoir. The system is engineered for the brake fluid level to be at the “Min” line when the brake pads need to be replaced. When the caliper pistons are pushed back to install new pads, the brake fluid level should go right back to the full line again.

So, if the brake fluid level is low, check the brake pads before topping off the fluid. If you top off the brake fluid first and then need to replace the brake pads, you will have to remove the fluid you just added.

YouTube: Toyota Prius Gen 3 Brake Fluid Check (3:52)

Moisture Content Testing

As shown in the Brake Fluid Specifications Table above, the moisture content of brake fluid causes a significant drop in the fluid’s boiling point. Being able to quickly, and accurately, notify the customer if their brake fluid should be changed due to excessive moisture content is a valuable service.

Electrical Conductivity (Indirect Testing)


Electrical Conductivity Brake Fluid Moisture Tester

The most inexpensive way to test the moisture content of brake fluid is to use an electrical conductivity tester. The tester then uses the conductivity results to calculate the percentage of moisture in the brake fluid.

While this method is somewhat popular due to the low cost of the tester ($10) and the ability to perform this quick check in the service drive, the results are not known for accuracy.

A more professional approach is to have a technician test the brake fluid in the service bay using a tester that directly measures the boiling point.

Boiling Point (Direct Testing)

Boiling Point Direct Testing

The most accurate way to test for moisture content is to actually heat up the brake fluid to the boiling point and take direct measurements. That is exactly how the OTC Brake Fluid Safety Meter works.

The tester draws a sample of brake fluid into the meter and then heats the fluid up until it boils. This takes about a minute. The meter then displays accurate boiling point data on the screen.

While this tester sells for about $370, it is truly an “industry standard” in the business. If a customer is looking on, this looks more professional that pulling a $10 tester out of your shirt pocket that looks like a pen.

YouTube: Checking For Moisture Content (2:55)

Corrosion Inhibitor Test

A High Level of Copper Due to Depleted Corrosion Inhibitors

Brake fluid contains a corrosion inhibitor package that prevents internal rusting of steel brake lines, calipers, and ABS (antilock brake system) components. Over time, the corrosion inhibitor package can “wear out” and the coolant starts attacking the inner liner of the brake lines which contain a high amount of copper.

Brake fluid test strips are available that test for the presence of copper in the brake fluid. A high level of copper is an indication the brake fluid’s corrosion inhibitor package is depleted and the brake fluid needs to be changed.

YouTube: Checking For Contaminated Brake Fluid (1:21)

Flushing / Bleeding Brake Fluid

Overview

Brake fluid should be changed based on the following conditions:

  • Manufacturer’s time/mileage specifications
  • High moisture content (moisture content tester)
  • High levels of copper (test strips)
  • To remove air from the system

Flushing and bleeding brake fluid uses virtually the same techniques and the same equipment. The difference is:

  • Flushing Brake Fluid – All the brake fluid in the system is changed (brake fluid in the master cylinder reservoir and in all four wheel circuits)
  • Bleeding Brake Fluid – Bleeding brake fluid focuses on just removing air from the system which is often introduced during a repair procedure. For example, if a tech is being paid flat-rate to change one brake caliper, the tech is only responsible for bleeding the air from that one caliper. If the customer wants all the brake fluid changed, that is a brake flush and involves a higher labor charge.

Manual Flushing / Bleeding

While manually bleeding brakes was common practice for many years, the advantages of pressure or vacuum bleeding have virtually obsoleted this practice. Two real world major disadvantages of manual bleeding brakes are:

  • It Requires Two People – While there is a single person technique, it tends to work better with two people. If both are on flat-rate pay, one isn’t getting paid during the process. It also requires a fair amount of communication and coordination between the two techs which isn’t always that easy to do in a noisy shop environment.
  • Potential Master Cylinder Failure – While pumping the brake pedal during manually bleeding, the master cylinder piston is pushed into a part of the master cylinder bore that may have sludge buildup. This can sometimes cut the master cylinder piston cup and kill the master cylinder. Who pays for this?

YouTube – Manually Bleed Brakes – One-Person Method (4:37)
YouTube – Manually Bleed Brakes – Two-Person Method (5:40)

Pressure Flushing / Bleeding

Pressure bleeding is the most expensive way to bleed brakes with most of the cost being in the master cylinder adapters. It is a viable solution for bigger shops that have a significant amount of brake bleeding work to do.

Typical Power Bleeding Steps

  • Use a siphon to remove all the old brake fluid from the reservoir (this saves having to push all the old fluid out at the wheel).
  • Refill the reservoir with new brake fluid
  • Attach the correct master cylinder adapter to the reservoir. (Direct fit adapters are preferred)
  • Attach a power unit to the master cylinder adapter.
  • Apply and adjust the pressure applied to the brake system
    • Electric Unit – Simply turn on the switch and adust with the pressure knob
    • Hand Pump Unit – Pump the unit by hand until the target pressure is reached.
  • Loosen the bleeder screw of the first wheel to bleed and catch the old fluid in a container. When the fluid color changes from dark to clear, tighten the bleeder screw and move to the next wheel in the sequence.
    • Start with the wheel that is the farthest away from the master cylinder and work your way back to the closest unless the manufacturer specifies a different sequence.

Adapters

While there are various types of pressure units, it starts with a good set of adapters. The cost of the adapters can easily exceed the cost of the actual pressure unit.

The kit shown to the left includes many “direct fit” adapters which attach to the master cylinder reservoir just like the original cap does. The kit also includes two “universal” adapters which are held onto the master cylinder reservoir with a chain. The universal adapters take more time to attach and tend to leak.

Power Units

The most inexpensive power unit is the manual pump style as shown in the image to the left. The pressure is created by manually pumping the unit.

The electric power unit, shown to the left and below, is preferred when doing a significant amount of power bleeding work.

YouTube – Hand Pump Bleeder Procedure (4:22)
YouTube – Electric Pump Bleeder Procedure (4:34)

Vacuum Flushing / Bleeding

Vacuum bleeding is becoming more popular over time. It does not require expensive adapters and most techs own their own unit.

Typical Vacuum Bleeding Steps

  • Use a siphon to remove all the old brake fluid from the reservoir (this saves having to pull all this old fluid out at the wheel).
  • Refill the reservoir with new brake fluid
  • Loosen the bleeder screw of the first wheel to bleed and use the vacuum bleeder to suck out the old fluid. When the fluid color changes from dark to clear, tighten the bleeder screw and move to the next wheel in the sequence.
    • Start with the wheel that is the farthest away from the master cylinder and work your way back to the closest unless the manufacturer specifies a different sequence.

Typical Vacuum Bleeding Units

The large unit shown to the left can be used for vacuuming brake fluid or even to vacuum out engine oil for an oil change. It can create a vacuum from shop air or can be manually pumped for mobile use without shop air. A large unit like this is typically bought by the shop

The smaller vacuum unit, as shown on the left at the bottom, works just as well but just in a smaller form factor. This is the type of unit often bought by technicians.

YouTube – Vacuum Brake Bleeder (10:31)