Brake Performance Blogs

When you step on your brake pedal, it forces the small piston(s) inside your brake caliper to compress, squeezing your brake rotors and essentially slowing down your car. Although this is the simplest way to explain the concept, it doesn't factor in some of the details that allow this to happen. Because the brake pedal doesn't just magically connect to all four brake rotors, it relies on the use of brake lines to divert your single force acting on the brake pedal onto the four corners of the car. These lines are filled with a hydraulic fluid we call brake fluid.

Brake fluid works well because it's a non-compressible, hygroscopic fluid which allows all the energy that your foot releases onto your brake pedal to be converted into braking pressure that squeezes your brake rotors and slows them down. So why is brake fluid so vital, yet often overlooked when shopping for brake rotors?

When you apply your brakes it causes your brake pistons to compress and clamp the brake rotors. This friction creates heat, lots of it. This means your brake fluid needs to retain its incompressible nature without boiling or evaporating due to the temperature.

What happens when your brake fluid boils or overheats? It creates air bubbles in the system and air is compressible opposed to brake fluid. So in turn, when you press down on the brake pedal, instead of clamping the rotors and slowing down the car, the brake fluid compresses the air and the brakes are not doing their job. This also translates to the driver as a mushy or unresponsive feeling brake pedal.

New hub units may exhibit rough feel when rotated by hand, a characteristic common with today's high performance products. SKF reports the rough or coarse feel of some new hub units if they are rotated by hand before installation and explains why these conditions are normal for technicians to experience.

In recent years there have been concerns about new hub bearings being rough or difficult to rotate. Many technicians inspect a new replacement hub bearing visually and by rotating the assembly once or twice by hand.

During the inspection process, it is not abnormal to feel a rough, coarse sensation as the two halves of the hub are rotated. This condition is not an indication of a defective bearing; these hub units should not be returned as suspected defective parts.

This condition is caused by the premium grease SKF uses in many of its bearings, especially in hub bearings. SKF uses GHG grease that provides a significant advantage over other standard greases.

SKF GHG contains many performance-enhancing additives, some of which are in the form of soft crystalline structures when the grease is new. These crystalline structures can create a rough, coarse feeling when the hub bearing is first turned by hand, before the bearing has experienced significant rotation.

These additives are important to optimum wheel bearing performance and include anti-brinelling and extreme pressure performance enhancers and solid lubricant components. This advanced technology grease chemistry improves bearing performance from many aspects, ranging from protecting against bearing damage during vehicle transport to maintaining proper lubrication conditions during high load/low speed operation to lubricant stability at elevated operating temperatures.

Even though the bearing may feel rough when it is initially rotated by hand, after rotating on the vehicle for a few minutes, the soft crystalline structures in the grease will break down into finer structures, and the bearing will rotate smoothly. There is no detrimental effect of the SKF GHG grease run-in. This is the normal and desired behavior of this type of grease.

Another item that may contribute to a hard turning hub bearing is the seal. In some hub bearings, the seals also have more interference with the bearing. Once the bearing is rotated for a few minutes, the bearing will gradually turn freely. This is a normal situation.

As a reminder, Generation 3 hub assemblies need to be fully preloaded/seated with the correct torque in order to operate properly. Without the proper bearing seating on a hub assembly, the bearing may have a feeling of roughness when turning it. The rough feel is more noticeable in bearings that are preloaded from the factory, as many of these newer generation hub units are preloaded. Always remember to follow the OE manufacturer's recommended torque specifications and procedures when installing the axle nut. This will create the correct preload for the hub assembly.

Brake fluid is a hygroscopic fluid that starts absorbing water, making it less effective. We suggest flushing your brake fluid once a year, but it's also a good idea to bleed your fluid every 6 months if you do a lot of hard braking. Hard braking on the brake rotors accumulates a lot of heat in the brake system and "boils" the fluid, creating air bubbles. Bleeding your brake system helps get rid of those air bubbles in the system.

For both bleeding and flushing your brakes, it's recommended to have fresh brake fluid readily available. If the fluid in the reservoir runs low while bleeding/flushing, it will induce air into the system, rendering the entire process useless.

Make sure the brake fluid is fresh and hasn't been sitting on the shelf more than 2 weeks if the seal has been opened. This is important as opened brake fluid is going to contain moisture. Unlike other brake components such as brake rotors or brake pads, brake fluid is sensitive to shelf life, especially when opened.

Always wear safety glasses and use proper equipment. Use common sense and take precautions as BrakePerformance does not take responsibility for erratic driving, accidents, damages done, or personal injury.

1. On a level surface, set the car in gear and place a stop behind the tires to keep it from rolling.

2. Open the hood and locate the master cylinder. This is where the brake fluid is filled from. Open the master cylinder cap (brake fluid reservoir) and place an old rag around it as you may get some spillage. (Do not drip on the paint of the car as brake fluid will etch.)

3. Using a turkey baster, suck out all the fluid. Most likely 20% will remain and that's okay.

4. Refill the reservoir back up to the fill line using fresh brake fluid.

5. Check with your manufacturer and find out the order in which brake calipers should be bled first. Normally, it's always starting with the furthest brake caliper from the brake reservoir. For example, if your reservoir is located in the engine bay by the driver side, you would start with the rear passenger brake caliper, rear driver brake caliper, front passenger brake caliper, and then finally the driver side brake caliper. This is just a general example; make sure to verify with your manufacturer.

6. Once you figure out the order, you want to jack up that side of the car and remove the wheel to access the caliper that you are about to bleed.

7. Secure the car by using jack stands once you jack up the car and remove the wheel.

8. Locate the bleeder valve. It helps to attach a rubber hose over it so the brake fluid doesn't drip everywhere.

9. Using an assistant, have him/her pump the brakes about 4-5 times, and they should notice it get stiffer. While they are holding the brake pedal down, open up the bleeder valve and fluid will come out. The trick is that you want your assistant to notify you to close the valve right before they hit the bottom of the brake pedal.

10. Repeat this process until fresh brake fluid comes out. Usually, old fluid is darker, and you will notice the shift to a lighter fluid indicating the fresh fluid has filled that brake line.

11. Repeat the same process for the remainder of the brake calipers and make sure that the brake fluid never gets below the minimum marker on the master cylinder. If this happens, you risk getting air into the system again and having to redo the entire process.

12. Once all four corners are done, top off the fluid in the reservoir to the max line, torque the wheel bolts to the proper specification, and verify you have a solid brake pedal feel before driving.

Bleeding Brakes

The process of bleeding brakes is identical. The only difference is that when you are bleeding each individual brake caliper, you only need to open the bleeder valve once or twice until you don't see any air bubbles come out. You do not need to wait until you start seeing fresh fluid. It is still necessary to have fresh fluid as it may drain most of what's in the master cylinder.

Brake fluid is a hydraulic fluid used in your brake lines. It takes the force your foot exerts onto the brake pedal and transfers it into braking pressure that then clamps your brake rotors, effectively slowing your car down. We use brake fluid due to its hygroscopic nature and its ability to resist compression. Essentially, you transmit all your force from the pedal to the brake rotor without losing energy due to fluid compression.

Now, because the brake rotors and brake calipers exert a high amount of heat, brake fluid must have a high heat tolerance. Otherwise, it would simply vaporize inside your brake lines and negate the purpose of the brake fluid. This is dangerous and results in your brake system not functioning properly.

Boiling Point is the maximum temperature before your fluid starts to boil. To further complicate things, there is a Dry Boiling Point and a Wet Boiling Point. Wet simply means the boiling point of a fluid after it has absorbed a certain percentage of moisture* (remember, it's hygroscopic). Dry is fresh fluid from a canister that hasn't been opened.

Below, we have the quality standards for each of the fluid classes:

• DOT 3 205°C/401°F (Dry) 140°C/284°F (Wet)
• DOT 4 230°C/446°F (Dry) 155°C/311°F (Wet)
• DOT 5** 260°C/500°F (Dry) 180°C/356°F (Wet)
• DOT 5.1 270°C/518°F (Dry) 190°C/374°F (Wet)

It's always recommended to only stick with the DOT rating your car recommends.

Using a DOT rating other than what is recommended may cause improper brake operation or premature failure.

*Wet boiling point is defined as 3.7% water by volume.

**DOT 5 is a synthetic non-hygroscopic fluid and should only be used for designated cars that support DOT 5.