Brake Fluid

  1. Basic brake fluid & fluid maintenance information.
  2. Why do I need to bleed my brakes?
  3. What is a slave cylinder?
  4. What is a brake booster?
  5. Can brake fluid damage brake pads?
  6. A word of caution about brake fluid!
  7. What is a master cylinder?
  8. How do I bleed my brakes?

Q: Basic brake fluid & fluid maintenance information.

Brake fluid is hygroscopic, meaning it absorbs water. When new from the bottle, it can be considered "dry" with a higher boiling point. Over time, brake fluid absorbs water lowering its boiling point to the "wet" level. Changing brake fluid removes water from the brake system and is extremely important. Change your brake fluid at least every two years … much more often for cars that see track use.For street cars, wet boiling point numbers are more important than dry because the fluid stays in your car for a long time (change it at least every two years). After months of exposure to humid air and changing climate conditions, brake fluid performance is closer to the wet boiling point than the dry boiling point. When brake fluid boils, it turns into vapor and forms air bubbles. This creates a “soft pedal” and requires excessive pedal travel to apply the brakes properly. Do not confuse this with "brake fade".  Brake fade occurs when the brake compound gets too hot from prolonged braking or improper bedding. Brake fade requires greater and greater pedal effort to stop the vehicle; boiling fluid increases pedal travel and makes the pedal feel soft.Due to the extreme temperatures that high performance brake systems operate at, standard brake fluids are not recommended for track use.Carbotech™ Performance Brakes does not sell and does not recommend silicone based brake fluid with any of its products. For more information, please scroll down to “A Word of Caution”.

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Q: Why do I need to bleed my brakes?

To remove moisture that entered the system. Brake fluid needs to be replaced periodically, because brake fluid will absorb moisture over time. This occurs whether a vehicle has been driven 60,000 miles, or has been sitting in a garage for a year. Fluid contamination is a function of time and humidity, and has nothing to do with the mileage driven. Moisture enters the brake system through seals and through microscopic pores in the hoses. Moisture enters the system every time the fluid reservoir is opened, which is a good reason not to open it unnecessarily.

To remove air bubbles that have entered the system because of a leak or because the fluid level got too low. The air must be removed because it is compressible and will result in a soft, “spongy” brake pedal.

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Q: What is a slave cylinder?

Slave cylinders (also known as wheel cylinders), are cylinders where movable pistons convert hydraulic brake fluid pressure into mechanical force. Hydraulic pressure against the pistons within the wheel cylinder forces the brake pads against the surface of the rotor. There is one cylinder (usually) for each individual wheel, which is built into the calipers.

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Q: What is a brake booster?

The modern brake booster is a brilliant device that operates using engine vacuum, engine vacuum is generated whenever the engines running. The brake booster takes engine vacuum via a rubber hose that runs from the intake manifold.  The brake booster uses that vacuum to amplify the pressure you put on the pedal ensuring that your car stops more rapidly.  What happens to the brake booster if your car stalls and loses it’s vacuum?  Engineers realized early on that gas engines were not foolproof, so they engineered a  check valve within the brake booster circuit. The brake booster stores enough vacuum to provide full boost for two or three pedal applications after the engine stalls. The check valve on the brake booster is what keeps that vacuum from leaking out. If the brake booster can’t hold vacuum its time is up and you’ll need a new or remanufactured brake booster.

Q: Can brake fluid damage brake pads?

ABSOLUTELY, that’s why it is so vital that the hydraulic system is in good working order with no leaks. This doesn’t mean that you have to throw away your brake pads.  Clean them compounds off and take them to a grinder (if needed) to grind off the contaminated compound material.  Please consult a professional before using a grinder, and to make sure the pads are safe to use again. 

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Q: A word of caution about brake fluid!

A Word of Caution …

Neither Carbotech Performance Brakes nor AP Racing markets silicone brake fluids nor recommends their use with any braking system.

Virtually all of the problems with silicone brake fluids reflect certain properties of silicone fluids identified by us over many years and recently ratified in SAE publications, specifically: high ambient viscosity; high air absorption; high compressibility; low lubricity; and immiscibility with water. Research has shown that these properties of silicone fluid result three performance problems:

Long Pedal Travel or “Spongy” Pedal caused by:

  1. High compressibility, up to three times that of glycol based fluids;
  2. High viscosity, twice that of glycol based fluids, leading to slow rates of fill and retention of free air entrapped during filling, and hence bleeding difficulties.

A Sudden Loss of Braking resulting from:

  1. Air absorption - gasification of absorbed air at relatively low temperature produces vapor lock effect;
  2. Immiscibility (failure to mix) with water - whilst the presence of dissolved water will reduce the boiling point of glycol based fluids, any free water in silicone-filled systems will boil and produce vapor lock at much lower temperatures (100°C or thereabouts)

“Hanging On” of brakes due to:

  1. Low lubricity - in disc brake systems the sole mechanism for normalization of system pressure upon release of pedal pressure is a designed-in tendency of seals to recover to their ‘at rest’ attitude. Low lubricity works against this tendency.
  2. High viscosity exacerbates the effects of low lubricity.

The high price of silicone fluids does not produce higher performance in hard driving or even normal road use.

AP Racing glycol based fluids do not exhibit the adverse properties of silicone fluids. The recently introduced Formula DOT 5.1, which exceeds the performance criteria of DOT 5 (Silicone), is suitable for all conditions likely to be encountered in modern driving conditions.

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Q: What is a master cylinder?

The master cylinder is a control device that converts physical pressure typically from a driver's foot applying pressure on the brake pedal, into hydraulic pressure to operate other devices in the hydraulic system. In brake systems, the operated devices are brake calipers.The master cylinder displaces hydraulic pressure to the rest of the brake system.

There are two separate systems that may be supplied by separate fluid reservoirs, or they may be supplied by the same reservoir. Most brake subsystems are divided front to rear. Some can be diagonally separated.

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Q: How do I bleed my brakes?

Bleeding Process: It takes two people

  1. First, always start at the corner furthest from the driver and work your way towards the driver/master cylinder (Right rear, left rear, right front, left front.), just remember farthest to closest.
  2. Then, find the bleeder screw (bleeder valve) on the rear caliper.  Attach a piece of clear tubing to the nipple of the bleeder screw.
  3. Place the other end of the hose into the disposable bottle.
  4. Then take a wrench and place it on the bleeder screw.
  5. Place the bottle for waste fluid on top of the caliper. Hold the bottle with one hand and grasp the wrench with the other hand.
  6. Next, have your assistant apply the brake pedal and hold down firmly.  Then open the bleed screw ¼ turn to release the fluid into the waste line.  NEVER have the driver release the brake pedal while the screw is open, make sure to CLOSE the screw first and then have the driver release the brake pedal.     
  7. Inspect the fluid within the waste line for air bubbles.
  8. Continue the bleeding process (steps 1 through 7) until air bubbles are no longer present. Be sure to check the brake fluid level in the reservoir after bleeding each wheel, make sure to keep the fluid level at the MAX line.
  9. Then, go on to the next corner and repeat all steps (1-8).
  10. Next, take brake clean and spray and whipe down the entire area until dry.  DO NOT SPRAY BRAKE CLEANER ON ANY RUBBER OR PLASTIC PARTS.  Leaving the area clean and dry will make it much easier to spot leaks. 
  11. Test the brake pedal for a firm feel, and if all the steps were followed then your done bleeding your brakes.
  12. Please discard of any and all used brake fluid properly and responsibly.

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