Brake Fluids and Bleeding Air

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Brake fluids in your car’s brake system is based hydraulic fluid used to transfer power from the brake pedal to the brake calipers or wheel cylinders. The purpose of the brake system fluid change is remove moisture, air and containment’s from the braking. Moisture is the leading cause of brake system operation failure .

Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly. Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disk brake calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization is a problem because vapor is compressible and negates hydraulic fluid transfer of braking force.

Because oils damage rubber seals and hoses in the braking system, brake fluids are not petroleum-based. Classification based brake fluid from the constituent material, Glycol-ether/dot three/dot four/dot five point one brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid’s service life, although at the cost of potential phase separation/water pooling and freezing/boiling in the system over time – the main reason single phase hygroscopic fluids are used.

Most automotive professionals agree that glycol-based brake fluid, (DOT 3, DOT 4, DOT 5.1) should be flushed, or changed, every 1–2 years to ensure reliability and safety. Electronic testers and test strips are commercially available to measure moisture content. The corrosion inhibitors also degrade over time. New fluid should always be stored in a sealed container to avoid moisture intrusion.

How to remove brake fluids :

  • Keep these facts in mind while handling brake fluid:

o Never use DOT 5 brake fluid in a vehicle designed for DOT 3 or DOT 4 fluids. It is ok to use DOT 4 fluid in a vehicle that requires DOT 3, but not vice-versa.

o NEVER substitute any other fluid for brake fluid.

o Wash your hands immediately after coming in contact with brake fluid. It eats paint, so imagine what it’ll do to skin.

o Properly discard brake fluid that’s been unsealed for more than one year. Check with your city for recycling days or centers.

Bleeding the Brake Lines to Remove Air

Two people are required to bleed a brake line; one to depress the brake pedal and one to drain the fluid into a container. These directions are the basic procedure for bleeding a brake line. Consult your service manual before you begin to bleed your own lines; this WILL NOT work for newer cars.

Have the following items handy:

o A section of plastic, vinyl or rubber tubing, 3/16" diameter, long enough to reach from the brake caliper to the clear plastic container.

o A wrench to open and close the bleeder valve.

o A clear plastic container containing at least one inch of clean brake fluid, or enough to submerge the plastic tubing. Clear plastic is important so you can see the air bubbles as they exit the brake line.

Here are the basic instructions for the process:

o Open the reservoir and add brake fluid up to the full line.

o Replace the cap on the master cylinder reservoir before you begin bleeding the line. Never remove the reservoir cap while the brake pedal is depressed.

o Starting with the wheel rear wheels, locate the brake cylinder or caliper and find the bleed valve on the back side. It will look line a bolt with a nipple on it and may have a rubber cap that you will have to remove.

o Place one end of the clear tube into the clear container to which you have added approximately one inch of brake fluid. Be sure to do this step before you connect the other end of the tube to the bleed valve.

o With the tube end in the container of brake fluid, connect the other end of the tube to the bleed valve.

o Use a wrench to loosen the valve on the brake caliper just slightly. Do not allow the brake fluid to begin to flow; just loosen the valve enough to make it easy to release when you are ready to bleed the line.

o Now, have the person inside the vehicle pump the brake pedal a few times to build up pressure in the line, and then have them hold the brake pedal down firmly, without further pumping.

o While the brake pedal is held in place, loosen the bleed valve enough to allow a small amount of brake fluid to flow out of the brake line and into your container.

o Watch for the air bubbles to exit the tube placed in the clear plastic container with the brake fluid.

o The person inside the vehicle should only allow the pedal to travel about 2/3rds of the way to the floor. Allowing the pedal to reach the floor may damage a master cylinder. Once the pedal reaches this position, the person inside should signals you with an "OK" so you know to tighten the bleeder screw. After the screw is tightened, signal the person inside with an "OK" so he may release the pedal.

o Repeat this action of pumping the brakes and bleeding the fluid until there are no more bubbles remaining in the line. It is very important to remember to periodically check the fluid level in the reservoir, and always replace the cap before actuating the brake pedal.

o Repeat the process above for the three remaining brake lines. Be sure your helper pumps and holds the brake pedal each time.

Once all four brake lines have been bled, fill the brake fluid reservoir to the "full" line, then close the cap.

Brake Fluids and Bleeding Air was last modified: August 15th, 2014 by tompul
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