Brake Fluid and Bleeding Sequence: ASE A5

ASE A5: Brake Fluid And Bleeding Sequence

The ASE A5 Test includes a portion on brake fluid, bleeding, flushing and leak testing. You must know how to:

  • Diagnose poor stopping, pulling, dragging, or incorrect pedal travel caused by problems in the brake fluid; determine needed repairs.
  • Bleed and/or flush the hydraulic system using manual, pressure, vacuum or gravity method(s).
  • Pressure test brake hydraulic system.

Looking for Contaminated Fluid

Brake fluid is a simple maintenance item, but one not to be overlooked. Contaminants such as water, rust, dirt, mineral oil or compounds from overheated glycol get into brake fluid and can cause damage throughout the braking system. Moisture can create bubbles within the brake lines that diminish stopping power, or it can cause corrosion within the caliper, which can seize pistons and eventually lead to a pull. Mineral oils can attack rubber seals and cause leaks; dirt, rust and debris can clog valves.

Remember that most brake fluid is a polyglycol formulation, which will absorb moisture. So, an open can of polyglycol brake fluid should be used immediately to avoid any inadvertent moisture absorption.

Check the condition of the fluid itself when checking fluid level. You want to see relatively clear fluid. Cloudy fluid is an indication of moisture, and dark fluid indicates debris. If the fluid looks layered, that indicates a silicone brake fluid was mistakenly mixed in with the polyglycol fluid. In any of these cases, the entire system should be flushed and refilled with the recommended fluid.

Brake Bleeding

Brake bleeding pushes new brake fluid through the brake system in order to force out contaminated fluid and trapped air that can enter the brake lines whenever the system is opened for service. Brake system bleeding must be done in a particular sequence:

1. Master cylinder
2. Combination valve
3. Wheel cylinders and brake calipers
4. Load-sensing proportioning valve
5. Antilock brake system (ABS) hydraulic modulator or pump motor.

Obviously, there are some caveats. The combination valve, load-sensing proportioning valve and ABS hydraulic modulator are not present on all vehicles, and the wheel bleeding sequence will always vary vehicle to vehicle. In some cases, you may have to re-center the pressure differential switch after bleeding the wheel brakes. Some disc brake systems also have a metering or combination valve that needs to be deactivated before bleeding because the operating pressure of power bleeders is such that it can trigger the fluid flow block to the front brakes.

Wheel Brake Bleeding

Follow the bleeding sequence recommended by the OEM. Generally, the wheel cylinder or caliper furthest from the master cylinder is bled first, followed by the next closest caliper or cylinder, and so on. In every wheel brake bleeding, fill the master cylinder with fluid and ensure that it stays at least half full during the entire procedure.

Manual brake bleeding

This method requires one person to press the brake pedal and another to open and close the bleeder valves. As the valve is opened, the brake pedal will sink toward the floor. Upon a signal, the bleeder valve should be tightened. This sequence repeats until all of the air is expelled. To manually bleed a system:

1. If the vehicle has a vacuum or hydraulic power booster, discharge it by pumping the brake pedal with the ignition OFF until the pedal feel hardens.

2. Place a clear plastic hose over the bleeder valve of the first wheel cylinder or caliper in the bleeding sequence, and submerge the open end of the tube in a partially filled container of fresh brake fluid.

3. Loosen the bleeder valve approximately one-half turn, while the other technician slowly depresses the brake pedal and holds it to the floor. Air bubbles leaving the bleeder valve will be visible in the hose to the container.

4. Tighten the bleeder valve, while the other technician slowly releases the brake pedal.
Repeat steps 3 and 4 until no more air bubbles emerge from the bleeder valve.

Transfer the plastic hose to the bleeder valve of the next wheel cylinder or caliper in the bleeding sequence and repeat steps 3 and 4. Continue around the vehicle in the specified order until the brakes at all four wheels have been bled.

Brake Fluid Q&A

Q: What are the fundamental differences between synthetic fluid and conventional DOT 3 and 4?

A: Simple, it all comes down to the base stock. Both synthetic and conventional brake fluids start from the same “polyethylene glycol” stock. But, to make synthetic brake fluid the manufacturer will “synthesize” the original base stock and alter the molecules. They could make the chains longer or add other molecules to enhance the performance. Almost every synthetic brake fluid manufacturer has a different approach.

To the base stock they add the additive package. This is a mixture of anti-corrosion, anti-foaming and other secret chemicals that give the fluid the desired performance characteristics for operation in certain vehicles.

DOT 5.1 is a high-performance certification (higher temps than DOT 5). To meet the performance criteria, it takes a synthetic or really good conventional base stock.

Silicone is a synthetic substance, but don’t confuse Dot 5 with synthetic polyethylene glycol-based fluids like DOT 5.1.

Q: What is the compatibility between synthetics and conventional DOT 3 and 4 in most vehicles? Is a complete flush with new rubber a better choice? Is there any chance of seal swelling or disintegration in older vehicles?

A: All DOT 3, 4 and 5.1 brake fluids are compatible with each other and with all systems. All polyethylene glycol-based fluids will not harm healthy rubber parts. Also, the additive packages will not damage or distort any rubber parts. Even an older rubber part with a high concentration of natural rubber will not be damaged by new DOT-rated brake fluids.

What kills rubber parts is when the additive package breaks down. The additive package controls the pH of the fluid and the viscosity. If the brake fluid becomes unable to control the pH or other corrosive elements, the rubber and metal parts will deteriorate over time. If you have an older vehicle and are worried about boiling the fluid, use DOT 4 or 5.1 brake fluid.

Q: Can synthetics be mixed where DOT 5 silicone was used or is it recommended that the rubber be replaced when changing back?

A: Silicone is an inert substance, this is why it is safe for breast implants. Technically, it should not damage the rubber parts.

Polyethylene glycol-based fluids (conventional or synthetic) will not mix with silicone fluid (DOT 5). It will lump together somewhere in the system. Also, they will not react when mixed. So, if you perform a good flush, the rubber parts and the system should be OK.

Q: Now that it’s known that copper contamination is the main reason that brake fluids degrade, what property do synthetics have to address this problem?

A: Copper comes from the copper brazing in the walls of the hard brake lines. Copper can be a problem for ABS and some valving, but it is an indicator of the state of the brake fluid. The more copper that is present, the greater the chance that corrosion is happening in the system. When there is corrosion, it’s a sign that the brake fluid’s anti-corrosion additives are depleted.

If your base stock and additives are of higher quality, chances are that they will not break down as easily as low-quality ingredients.

Q: Compared to conventional fluid what life span can be expected from synthetics?

A: Synthetic fluid might be able to last a little longer because the fluid can absorb more water before the fluid drops below a critical boiling point. But, it is the additive package and environment that has the greatest impact on life span, no matter what type of base stock.

Q: Some have said that they have noted a better pedal feel with synthetic fluids. Is this possible?

A: Some very high-end synthetic DOT 4 and 5.1 synthetic fluids can give a better pedal, but it is so small of a difference that only the very best drivers can feel it. It’s most likely in their head.

Some silicone-based fluids can give a softer pedal because they are more compressible than glycol-based fluids, but high-tech silicone ester-based stuff that some race teams use is less compressible than glycol-based fluids. This stuff can run $90 a liter. Also, it does not have some critical corrosion inhibitors and is not DOT certified.

DOT 5 brake fluids still have their place. Owners of show cars can use the fluid if they are concerned about damage to the paint.

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