How to Fill Your Master Cylinder

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All master cylinders are marked with the correct type of brake fluid for that system. Learn how to fill your brake master cylinder in this free auto repair video featuring a certified mechanic.

Part of the Video Series: How to Rebuild Front Brake Calipers
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Video Transcript

In this clip, we're going to talk about the proper service procedure to rebuild a front brake caliper. In this step, we're going to talk about the proper method used in determining the brake fluid we're going to use and filling up our master cylinder. Now, all master cylinders are supposed to be marked with the type of brake fluid that it takes. If yours has been damaged or replaced, refer to your vehicle's owner's manual to know which kind. There are three kinds of brake fluid, most commonly available. This is dot three. It's what this vehicle is designed to operate on so it came in when it was new from the factory. They also offer dot four and dot five. As they increase in number, they increase in boiling point and hygroscopic ability. The boiling point is exactly what temperature the brake fluid will boil at and the higher, the higher the temperature thus making a better fluid for braking. The hygroscopic number is the amount of moisture that it could absorb from the atmosphere. Having moisture inside your brake lines is extremely bad. It causes corrosion and will boil at a lower boiling point than your brake fluid. Dot three can only be used on dot three vehicles. That's all it's specified for. Dot four can be used on both dot four and dot three vehicles. Dot five can be used on dot five, dot four and dot three vehicles. You can use a higher number for a lower, but not a lower number for a higher number. Once you've determined the correct kind of fluid to use, you're going to want to make sure that your master cylinder cap and reservoir are clean of debris and foreign material. We're going to take and unscrew the cap. If your reservoir boot has extended during the bleeding process, you're going to take and push this back in with your finger. It will just pop in and out. We're going to sit this aside. The master cylinder will have a maximum fill level on it. As you can see, it's indicated by the letters M A X and this line right here. We are a little low. What I'm going to do is take and add brake fluid until we achieve that maximum fill level. Right there. You're also going to want to make sure that the float inside the reservoir is free floating. Just take and touch it with your finger, right there. It is possible, it's fairly common for those floats to stick triggering your red brake light to stay on. Once you've filled it to the max level with your appropriate fluid and made sure the float is not stuck, that is the proper service procedure for filling your master cylinder.


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