Auto Repair For Dummies, 2nd Edition
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Essentially, when you check your master cylinder, you’re making sure that you have enough brake fluid, which is stored in the master cylinder. When you step on the brake pedal, fluid goes from the master cylinder into the brake lines; when you release the pedal, the fluid flows back into the master cylinder.

To check the brake fluid in your master cylinder, follow these steps:

Open the brake fluid reservoir on top of your master cylinder.

Open the brake fluid reservoir on top of your master cylinder.

If you have the kind with a little plastic bottle on top, just unscrew the cap on the little plastic bottle that sits on top of the master cylinder. If you have a metal reservoir, use a screwdriver to pry the retaining clamp off the top.

Don’t let any dirt fall into the chambers when you open the lid. If your hood area is full of grime and dust, wipe the lid before you remove it.

Take a look at the lid.

Take a look at the lid.

As the brake fluid in your master cylinder recedes (when it’s forced into the brake lines), the diaphragm cups are pushed down by air that comes in through vents in the lid. The cups descend and touch the surface of the remaining brake fluid to prevent evaporation and to keep the dust and dirt out. When the fluid flows back in, the cups are pushed back up.

If your brake fluid level is low, or if the cups are in their descended position when you remove the lid, push them back up with a clean finger before you replace the lid.

Look inside the master cylinder.

The brake fluid should be up to the “Full” line on the side of the cylinder or within 1⁄2 inch of the top of each chamber. If it isn’t, buy the proper brake fluid for your vehicle and add it until the level meets the line.

Close the brake fluid reservoir as quickly as possible so that oxygen or water vapor in the air doesn’t contaminate the fluid. And try not to drip it on anything; it eats paint!

If both chambers of your master cylinder are filled with brake fluid to the proper level, close the master cylinder carefully, without letting any dirt fall into it.

Because most master cylinders are pretty airtight, you shouldn’t lose brake fluid in any quantity unless it’s leaking out somewhere else.

Use a flashlight to look for stain marks, wetness, or gunk under the master cylinder.

If your master cylinder is — or has been — leaking, you’ll see evidence of it when you look closely.

About This Article

This article is from the book:

About the book author:

Deanna Sclar is an acclaimed auto repair expert. She has appeared on hundreds of radio and TV shows, including NBC's Today show and the NBCNightly News. Sclar lectures internationally on the ecological impact of vehicles and is active in promoting residential solar energy programs. Sclar is also the author of Buying a Car For Dummies.

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