Bench bleeding your master cylinder is not necessarily all that complicated, but if we can make it even easier for you…we will. What is bench bleeding a master cylinder you might ask and why should I do it? It’s a process done before starting the process of bleeding the brakes on the rest of the vehicle. Typically done on the work bench (hence the name), this will remove the air from the master cylinder which then speeds up and makes bleeding the rest of the system much easier.
People have done the tried and true method of using two plastic tubes, a fitting and a screwdriver for years. It isn’t wrong and quite frankly, it works. We feel however that there is a simpler and better method. Our solution is our Bleeder Syringe Kit (Part# AC20010K). At first glance it looks like a very basic plastic syringe with a rubber nipple on the end. Your eyes are not deceiving you. It is a very simple tool and a very simple process but one that is sometimes misunderstood.
Begin by securely mounting your master cylinder in a vice or something similar to hold the master cylinder flat so fluid doesn’t pour out. Then fill the master’s reservoirs with your brake fluid of choice (Master Power Brakes recommends Pentosin Super Dot 4 Brake Fluid - Part# ACJ1703). Fill each reservoir about ½ full. Using the syringe, draw fluid from the reservoir. It is important to have enough fluid in the reservoir in order to draw enough brake fluid to fill the syringe to 50%.
Firmly press the rubber nipple to the port making sure that a complete seal has been created between the fitting port and the nipple. As you slowly inject fluid into the exit port, you will see the air bubbles come to the surface inside the reservoir. Maintain a slow application of pressure to the syringe until you no longer see bubbles. Once a steady stream of fluid can be seen, the syringe can be pulled away from the port. Be sure to have a damp towel below the master cylinder available as fluid will be in the port area and will run out. The plastic plugs that are provided with the syringe kit can be used to plug each the port. Repeat the same process in the other port on the master cylinder.
Once this procedure has been done to all of the ports that will be used in the master cylinder, the master cylinder can be considered bench bleed and ready to install on the vehicle. We have discussed this process being done on a work bench. In reality, this can also be done on the vehicle if room permits. Take extra precaution if doing it on the vehicle so brake fluid doesn’t damage any painted surfaces.
The process is that simple. The great thing is you can clean the syringe up and put it in your toolbox and use it as many times as you want. We are confident that you won’t bleed a master cylinder a different way ever again.