Replacing brake fluid

Part 1. Preparation and safety

Objective

• Test and replace brake fluid in accordance with manufacturer's recommendations.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

• Work clothing - such as coveralls and steel-capped footwear
• Eye protection - such as safety glasses and face masks
• Ear protection - such as earmuffs and earplugs
• Hand protection - such as rubber gloves and barrier cream
• Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

• Never use any petroleum or mineral based products, such as gasoline, kerosene etc, to clean a braking system or its components. They are not compatible and will result in a failure of the braking system and its components. This may result in injury to the passengers or damage to the vehicle.
• If brake fluid splashes into your eyes, rinse it out with tap water immediately.
• Do not swallow brake fluid. It is toxic.
• Make sure that when you are bleeding the brakes, you do not empty the brake fluid reservoir or you will cause internal damage to the master cylinder.
• Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

• Brake fluid can damage the paintwork by softening the base paint.
• Brake fluid has a hygroscopic nature; it will absorb moisture rapidly.
• As moisture is absorbed, the boiling point is lowered. This can cause the phenomenon known as 'brake fade'.
• Always replace the reservoir cover or lid as soon as possible.
• Do not use brake fluid that has been left in an open container.
• Brake fluid types are identified by their D.O.T. number. The higher the D.O.T. number, the higher its boiling point.
• Most vehicles use either D.O.T. 3 or D.O.T. 4 fluid. These fluids are both glycol-based but differ in specifications. The reservoir cover or cap will usually state the type of fluid to be used.
• A D.O.T. 5 is synthetic fluid and normally referred to as 'silicone' brake fluid; it's usually colored purple for easy identification and it's not recommended for vehicles fitted with ABS.
• Do not mix D.O.T. 5 brake fluid with a D.O.T. 3 or 4 glycol-based fluid. They are incompatible.

• The majority of today's braking systems use a combination of dissimilar metals. Manufacturers use aluminum in pistons and housings, steel in some wheel cylinders and brake lines.
• When moisture mixes with brake fluid a galvanic action can occur. Galvanic action is similar to the operation of a battery. The more moisture in brake fluid, the higher the galvanic reading and the greater the corrosion it causes.
• If you need to bleed your brakes, you may need to replace your brake fluid.
• When bleeding the brakes, it will be necessary to top up the brake fluid after bleeding each brake station.
• Most manufacturers recommend that brake fluid be changed at least every two years. Consult your owner's manual for the specified intervals for the model you are working on and the type of replacement brake fluid recommended.
• There are a number of different brake bleeding methods. Some repair shops will use a 'pressure bleeder' while others may have a 'vacuum brake bleeder'. Manual bleeding is discussed in this procedure.
• Manufacturers recommend that when bleeding brakes; push the brake pedal to the floor. If you are changing brake fluid on an older vehicle, or if you are uncertain of the vehicle's service history, it is wise to depress the pedal through its normal range of movement. This will prevent the components in the master cylinder from coming into contact with contaminants.
• Brake assemblies, either disc or drum, are often referred to as braking stations.

Part 2: Step-by-step instruction

1. Check for galvanic reaction
   Remove the brake fluid reservoir cap, and set the DVOM to read voltage on the lowest voltage range of the meter. Place one of the DVOM probes into the brake fluid, and the other against the body of the master cylinder. The voltage should be no greater than 0.3 volts. If the voltage is higher, this indicates a galvanic reaction, and means that there is an unacceptable level of moisture in the brake fluid.
2. Remove the excess brake fluid
   Use an old hydrometer or similar device to suck out the contaminated fluid from the master cylinder reservoir. Place the discarded brake fluid into a container that can be disposed of in an environmentally friendly manner.
3. Top up with new fluid
   Top up the master cylinder reservoir with clean fluid until it reaches the “FULL” line. Close the brake fluid container as quickly as possible to prevent contamination and evaporation of the fluid.
4. Manually bleed the brake system
   Open the bleeder screw that is the furthest from the master cylinder and ask an assistant to slowly push the brake pedal down. With a clear bleeder hose inserted into a jar, you can observe the air bubbles coming out. Close the bleeder screw off, and have the assistant slowly release the pedal. Continue this process until there are no more air bubbles coming out of the brake station and the new brake fluid is present. Close off the bleeder screw and tighten it to the manufacturers specifications. Repeat this procedure for each of the other brake stations, moving closer to the master cylinder.
5. Inspect and test
   Have your supervisor inspect each of the brake stations to ensure that there are no leaks and the bleeder screws are closed correctly. A simple way of doing this is to ask the assistant in the vehicle to place their foot on the brake pedal, as if they were applying the brakes when driving the vehicle normally. Now you can check all the bleed points and hose connections for leaks while the assistant is maintaining the pressure on the system, and thus any fluid that could be leaking as a result of a loose joint in the components. After your supervisor is satisfied that it is all correct, lower the vehicle. Carry out a final check for any leaks around the master cylinder or brake stations. Replace the master cylinder cover and check to see that it is fully seated. Dilute any brake fluid that may have been spilt with fresh clean water. Ask your supervisor to check the brake system pedal pressure, fluid levels, and visual signs of leaks. When the supervisor is satisfied, the vehicle should be road tested by a qualified person to ensure the integrity of the braking system.