Tech Tip: Bleeding By The Numbers

First of all, what does “Bleeding the Brakes” refer to? To put it simply, it is the procedure used to remove air from the brake fluid in a brake system. Air can become trapped within the system at several points during the brake system’s life. The obvious and most common point is when the system is being assembled, to begin with, and the ports and the lines in the system are open and become filled with air.

The second is usually down the road once the system is closed up and the vehicle has been driven. Should the fluid reach a point of boiling under use, a vapor will be created that will act and be the same thing as air. This usually occurs under a very hard-driving situation.

The Importance

Why is it important to bleed the brakes? Unlike the brake fluid itself, air can be compressed when pressure is applied to it while brake fluid can not be compressed. The ability to compress air is what causes the feeling in the brake system of a soft or squishy brake pedal that is very undesirable when driving. In addition, if the air is left in the system at install or not removed later on down the road, the brakes may never actually work at all and could potentially fail while driving.

Before we start addressing the bleeding procedure, there are some things that need to be thought about and even discussed. Before beginning the bleed process on the brake system, it is always recommended that the master cylinder be bench bled before doing the rest of the system. It will save a bunch of time and headaches. Our Syringe Bleeder Kit is a fantastic tool for this step and will make life easier. To learn more; check out our Blog Post on how to use it.

First Tip

Our first tip is for people that are using a Combination Valve. If this applies to you, be sure to put the Bleeder Tool in place of the Pressure Differential Switch to eliminate any chance of the valve moving. For more information on the tool, check out our Tech Tip article dedicated to it. Our next tip talks about the calipers being used and the bleeder screws that might be present. Some calipers may only have a single bleeder screw while others will have multiple bleeder screws. For example, our Rallye Series calipers have four bleeder screws per caliper. This allows us to use the same caliper on the driver and passenger side. To bleed the caliper, you will only need to concern yourself with the bleeder screws that are pointing vertically. Since air is going to find it’s way to the top, the bleeder screws pointing downward will not do anything in the bleeding process. If there are multiple bleeders that are pointing upward, you will want to bleed from each bleeder screw in the process.

Now that the importance of brake bleeding is known and understood a little better, it is time to discuss the best practice for bleeding the brakes. There are many different methods out there along with different theories on how to best bleed a brake system. Some of the options involve a vacuum while others involve basic reversing of the flow of the fluid back into the master cylinder. We are not here to dispute any of those options. We are merely here to give you an option that has worked the best for us and what we feel is the best option.

The method we use day to day is the same one that was taught to most of us as young kids while in the garage working on cars with our dads. It simply involves a couple of people (one to be at the wheel and the other to push the brake pedal), a clear plastic hose that fits snugly to the bleeder screw, and a quart-size bottle.

First, starting at the passenger side rear of the vehicle which is usually the point furthest away from the master cylinder, locate the bleeder screw on the caliper (or wheel cylinder) and determine the hex size of the bleeder.

Put the wrench on the bleeder and then push the clear plastic hose on the end of the bleeder which should be fairly snug. The other end of the hose can be placed in the quart-size bottle with enough brake fluid in the bottom to allow the hose to be submerged in fluid.

With your extra body in the driver’s seat, have them make one slow push of the brake pedal to the floor and hold pressure on the pedal. With that pressure being held, open the bleeder.

Brake fluid should move through the plastic tube and air bubbles should be visible while the person pressing the pedal should notice the pedal moving downward. After a brief second, close the bleeder and have your helper release the brake pedal.

Repeat this same procedure several times at this wheel until all of the air bubbles have been removed. Once you have just fluid moving through the hose, move to the next closest which is usually the driver-side rear.

Once there, repeat the same procedure. It is worth mentioning that the brake fluid should be checked and filled regularly. Sometimes it is necessary to complete more than one round of bleeding at each wheel. This all depends on the brake system.

With the brakes completely bled and all of the air removed, using the proper cleaner of choice, make sure that all of the brake fluid has been removed from around the bleeder screws. This simply helps to clean things up so one can identify leaks that might be present later.

With that completed, it is time for a road test. Take it easy on the first applications of the brakes, especially with the car moving.

The only question remaining is how often should the brakes be bled. In a street-driven car, you should only have to bleed the brakes upon initial installation of the brake system. After that, the system should not require any additional bleeding of the brakes.

However, if the system is opened up for the replacement of anything, a full bleed should be performed as air will enter the system at that point. The only contradiction of not needing to bleed the brakes again would be if the car is tracked or hot-lapped. That will put the brake fluid in a situation where brake vapor is created. A maintenance schedule should be employed that gets the brakes bled at least once a year.

If you have any questions on how to bleed the brakes in your classic car, please don’t hesitate to give one of our technical representatives a call at 1-888-249-9425. We will be glad to walk you through any questions you might have.