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How to Bleed Vehicle Brake Systems



Purpose:

Proper bleeding of the brake system is critical to the performance of any vehicle. Without good quality disc brake pads, brake shoes, rotors, drums, clean fresh brake fluid and a properly bled brake system, the brake system cannot function properly. There are many misconceptions about bleeding brakes and brake fluid in general. The purpose of this article is to help dispel some of these myths and outline a reliable and repeatable method for bleeding the brake system.

Brake Fluid 101

There are 2 types of brake fluids commonly available today and used in disc brake pad systems. Glycol-ester blends (glycol-based) and high percentage ester content blends (ester-based). More common are the glycol-based. These glycol-based fluids normally have low compressibility but are hygroscopic in nature. This means they are able to absorb water from the atmosphere. This contamination process occurs whenever brake fluids are exposed to the atmosphere, and even occurs somewhat through the plastic bottles most brake fluids are packaged in. This is the reason that all brake fluids must be used fresh. The susceptibility of brake fluids to hygroscopic contamination can be judged by comparing the dry boiling point and wet boiling point of the fluid: the greater the difference is between these two ratings, the more hygroscopic is the nature of that fluid. This concept is important, as it is the absorbed water that gasses off (boils) first inside the brake system and adds to the spongy/soft/long pedal effect. Pay no attention to the large numbers used by the marketers to describe the product, as they can be very misleading. The dry boiling point of glycol-based brake fluids seldom exceeds 580F (304c), even though you'll see 600, 610 and 660 as a label description. The use of PFC caliper temperature stickers, p/n 032.0007 is highly recommended to monitor brake fluid and caliper seal condition. If the disc brake pads heat and calipers exceed 430f (210c) for an extended period, the brake fluid and or caliper seals will deteriorate.

High percentage ester blends have the promise of higher dry and wet boiling points. The ester base used in manufacturing these fluids is not hygroscopic at all. However, they are blended with a percentage of glycol for several reasons including cost (ester is more expensive to manufacture than glycol), pedal feel (ester is by nature more compressible than glycol), lubricity and seal conditioning. The most common ester-based brake fluid on the market, and the established standard in the racing industry, is Castrol SRF. It has a proprietary formulation and effectively combines the best properties of the ester base (stability at elevated temperatures and low hygroscopic characteristics) with compressibility near that of glycol-based fluids. Not all high ester blends are equal. In many cases, although they may have less water content to boil, compressibility is not as good at elevated temperatures, and the soft/spongy/long pedal effect is the same. Regardless of which brake fluid is used, frequent and correct bleeding should be considered part of proper disc brake pad service.

Brake Bleeding How To

As with any disc brake or brake shoe system on a vehicle, the brake system must be inspected and found to be in good working order before beginning. Inspect the entire brake system for leaks worn disc brake pads or brake shoes, and damaged parts including the pedal assembly. Inspect the master cylinder reservoir cap(s) to see that they are venting properly, as this is an overlooked and common cause for poor pedal feel. Once the disc brake pads and other components in the system have been inspected and serviced as necessary, begin the bleeding process by filling the master cylinder reservoir with clean, fresh, high quality brake fluid. Depending on the type of master cylinder arrangement, follow the instructions below:

    Notes on Bleed Bottles:
  • There are many brake bleed bottles commercially available for a reasonable price. It is helpful for the bleed bottle to be clear so a visual inspection of the brake fluid being purged is possible. The use of clear hose that fits tightly around the bleed screws adds a visual aid to bleeding then just relying on what is seen through the bleeder bottle. For many disc brake calipers, this hose should have a 1/8" ID or another appropriate size to fit the bleeders properly. Insure that the bleed hose is inserted deeply enough into the bottle so that the end is submerged in brake fluid to help close the loop. Bleed bottles should be vented as well. Discard spent brake fluid in a approved manor.
    Notes on Master Cylinder Fluid Reservoirs:
  • The master cylinder fluid reservoir must be positioned higher than the disc brake caliper bleed screws to insure proper bleeding. This means that if a floor-mounted pedal is used, the fluid reservoirs should be remote mounted above the level of the disc brake caliper bleeders. If this is not practical, alternative methods must be employed to bleed the disc brake calipers properly.
  • The fluid volume required for a master cylinder reservoir is based upon the maximum piston displacement of the disc brake calipers being used. Consult your disc brake pad or caliper manufacturer for this information. If there is insufficient fluid volume in the reservoir for the piston displacement required, the master cylinder will be empty before the disc brake pads are worn to their maximum. Total brake failure can result in this extreme disc brake pad situation. Particular attention must be paid to this in customized and racing vehicles.

Bleeding Procedure for Most Production Based Vehicles with Single Master Cylinder

    Supplies needed
  • Two people
  • Supply of high quality brake fluid from an unopened container
  • Brake bleed bottle
  • Six point box end or line wrench for bleed screws on calipers
  • With the car on jack stands and a person sitting in the drivers seat:
  • Place the wrench over the bleed screw on the caliper furthest from the driver.
  • Place the bleed hose attached to the bleed bottle over the bleed nipple with the wrench. If the disc brake caliper has two bleed screws, start at the inboard bleeder first.
  • Open the bleeder.
  • With the bleed screw is open, have the assistant slowly depress the brake pedal to the floor. With the driver holding the pedal down, gently close the bleed screw. With the bleeder closed, the driver can release the brake pedal.
  • Repeat this process until the fluid coming from the caliper is clear and free of air bubbles.
  • For calipers with 2 bleeders, repeat this process for the outboard bleeder.
  • Monitor the fluid level in the reservoir and refill as necessary during the process.
  • When finished torque bleeders to factory recommended settings

Repeat these steps for each disc brake caliper, working towards the disc brake caliper closest to the driver. At times it is helpful to tap the caliper body with a soft mallet to release any small air bubbles trapped inside the caliper. At this point, the brake pedal should be checked for firmness and consistency. If the fluid is free of air bubbles and the pedal is still soft or spongy, there may be further issues that need to be addressed with the brake system. When finished, fill the brake fluid reservoir to the full line, but not over.

Alternate Bleeding Methods

The methods outlined above are not intended to be the sole methods approved for bleeding brake systems. However, they are reliable and repeatable techniques designed to minimize the cavitation that occurs when fluids move too rapidly through small passages and orifices. Bleeding brakes is not a pressure dependent process; it is a flow dependent process. All that is required is the slow and steady evacuation of contaminated fluid and air from the system. This will enable the disc brake pads to engage and properly stop the vehicle.

Power Brake Bleeders

Power brake bleeders operate through applying external pressure to the brake fluid reservoir (pressure bleeders). If a power bleeder system is used, be sure a bellows or rubber diaphragm is used where the power bleeder applies it's pressure to the reservoir area. This reduces water or other contaminants from affecting the brake fluid. Vacuum Bleeders apply a vacuum to the bleed screw at the calipers and draw the fluid out the system. These are typically used in car and truck repair shops and are not generally considered acceptable for racing applications.

Closed Loop System Bleeding

Closed loop systems recirculate the brake fluid from the calipers back to the master cylinder reservoir via a series of check valves when the brake pedal is depressed and released. These systems are sometimes used in situations where gravel and debris continually damage bleed screws and other caliper appendages. This does not obviate the need to bleed the brakes, as the brake fluid becomes contaminated, just as in a conventional disc brake pad system. If a closed loop system is used, then 100% of the old fluid must be removed before bleeding these types of systems. If the fluid is not replaced frequently, the entire system will become contaminated and failure will result.

Brake Fluid Disposal

All brake fluids, disc brake pads and brake shoes as well as other components must be returned or disposed of properly. Brake fluid's ability to combine with water means it can very easily contaminate ground water supplies if disposed of improperly.

This documentation was provided by Performance Friction.

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