Brake system bleeding procedure

Purpose:
Proper bleeding of the brake system is critical to the performance of any racecar. Without good quality, 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. 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 can 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 calipers exceed 430°F (210°C) for an extend 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 PFC’s RH665. 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. PFC recommends PFC RH665 racing brake fluid because of decades of proven consistency and performance. However, 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 race preparation.

Brake bleeding – How to:
As with any system on a racecar, the brake system must be inspected and found to be in good working order before beginning. Inspect the entire brake system for leaks or damaged parts including the pedal assembly (refer to the Balance Bar setup guide). 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 all the 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 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 PFC calipers, this hose should have a 1/8” ID to fit the PFC bleeders properly. Ensure 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 an approved manner.

Notes on Master Cylinder Fluid Reservoirs:
• The master cylinder fluid reservoir must be positioned higher than the caliper’s bleed screws to ensure proper bleeding. This means that if a floor-mounted pedal is used, the fluid reservoirs should be remote mounted above the level of the caliper bleeders. If this is not practical, a “closed loop” system must be used, or the calipers should be dismounted and held to a lower location, below the reservoirs for bleeding.
• The fluid volume required for a master cylinder reservoir is based upon the maximum piston displacement of the calipers being used. Consult your 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 pads are worn to their maximum. Total brake failure will result. Particular attention must be paid to this in Endurance Racing events.

Production Based Vehicle with Single Master Cylinder

Supplies needed
• 2 people
• Supply of high-quality brake fluid from an unopened container
• Brake bleed bottle (PFC p/n 026.0001)
• 6-point box end or line wrench for bleed screws on calipers

With the car on jack stands and a person sitting in the driver’s 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 caliper has two bleed screws, start at the inboard bleeder first.
• Open the bleeder.
• With the bleed screw open, have the “driver” 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 caliper, working towards the 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 fluid reservoir to the “full” line, but not over.

Vehicle with Dual Master Cylinder and Racing Calipers

Supplies needed
• 3 people
• Supply of high-quality brake fluid from an unopened container
• 2 brake bleed bottles
• 2 6-point box end or line wrenches for bleed screws on calipers

Important Notes
It is imperative that both ends of the car be bled simultaneously when using dual master cylinders. If this is not done, the master cylinders will not be adequately purged of contaminated brake fluid. Further, the balance bar assembly may be severely damaged, leading to a failure of the brake system.

With the car on jack stands and a person sitting in the driver’s seat:
• Place a wrench over the inboard bleed screw on one front and one rear caliper.
• Place the bleed hose attached to the bleed bottle over the bleed nipple with the wrenches.
• Open the bleeders.
• With the bleed screw is open, have the “driver” slowly depress the brake pedal to the floor. With the driver holding the pedal down, gently close the bleed screws. With the bleeder closed, the driver can release the brake pedal.
• Repeat this process until the fluid coming from the calipers is clear and free of all air bubbles.
• Repeat this process for the outboard bleeders.
• Monitor the fluid level in the reservoirs and refill as necessary during the process.
• When finished, torque bleeders to factory recommended settings

Repeat these steps for the remaining calipers, bleeding the front and rear circuits simultaneously. 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 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.

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 its 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 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 commonly used in rally competition 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 brake system. If a closed loop system is used, then 100% of the old fluid must be removed before bleeding this type of system. If the fluid is not replaced frequently, the entire system will become contaminated, and failure will result.

Brake Fluid Disposal
All brake fluids must be disposed of properly. Their ability to combine with water means they can very easily contaminate ground water supplies if disposed of improperly.