Checking The Brake System of Your Master Cylinder


Essentially, when you check your master cylinder, you’re being sure that you have enough brake fluid, which is stored in the master cylinder. Once you step in the brake pedal, fluid goes from the master cylinder in to the brake lines; when you release the pedal, the fluid flows into the master cylinder.

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


1Open the brake fluid reservoir on the top of your master cylinder.

Just unscrew the cap in the little plastic bottle that sits on top of the master cylinder if you have the kind after some plastic bottle on top. Utilize a screwdriver to pry the retaining clamp off the top if you have a metal reservoir.

Don’t let any dirt fall into the chambers when you open the lid. Wipe the lid before you decide to remove it should your hood area is full of dust and grime.


2Take a look at the lid.

Since the brake fluid in your master cylinder recedes (when it’s forced in 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.

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

3Look inside the master cylinder.

The brake fluid should be around 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 within the air doesn’t contaminate the fluid. And try not to drip it on anything; it eats paint!

4If 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.

You shouldn’t lose brake fluid in almost any quantity unless it’s leaking out elsewhere, because most master cylinders are pretty airtight.

5Use a flashlight to search for stainmarks and wetness, or gunk under the master cylinder.

You’ll see evidence of it whenever you look closely if your master cylinder is – or has become – leaking.

It’s a good idea to check your master cylinder no less than every couple of months – more regularly if it was low in fluid when you last checked it.