How To Replace Your Master Cylinder

One of the most important components of the braking system is the brake master cylinder, which pressurizes hydraulic fluid to be sent to your vehicle’s calipers and wheel cylinders. If you notice a lack of brake system pressure, brown or black brake fluid, leaks near the master cylinder, or a brake pedal that slowly drops all the way to the floor, your car, truck, or SUV’s brake master cylinder may be failing. A damaged, leaking, or faulty master cylinder is a dangerous problem, and should be repaired before the braking system is safe while driving. This detailed video can help you perform a brake master cylinder replacement and you maintain your car, truck, or SUV’s brakes

Tools & Materials

Safety Eye Glasses Safety Glasses
Gloves Nitrile Gloves
chocks for wheels Chocks
brake line flare wrenches Brake Line Flare Wrenches
crescent wrench Crescent Wrenches
socket set Ratchet & Sockets
brake master cylinder Master Cylinder
O'Reilly Dot 3 Brake Fluid Brake Fluid
Siphon Tool Siphon Tool
drain pan Drain Pan
shop towels Shop Towels
O'Reilly Brake Cleaner Brake Cleaner

Brake Master Cylinder Replacement

Any time you do work on your brake system, pay attention to how your brakes function before and after the job to make sure the problem has been corrected. Also, be very careful handling brake fluid. It is highly corrosive and will damage any painted surface. If you’re having severe symptoms or aren’t completely sure about diagnosing this yourself, we’d be happy to recommend a professional technician in your area.


We'll be working on a 2008 Dodge Nitro, but the basic procedure will be similar on most vehicles with power brakes. Make sure to know the specifics for your vehicle before getting started.


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Propping up the hood of truck and disconnecting car battery terminal

Step 1: Prepare Your Vehicle

Chock your wheels and set the parking brake. Pop the hood and disconnect the negative battery terminal. Set it aside someplace where it isn’t touching metal.
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blue gloved hand pointing out vacuum line

You’ll need to relieve the vacuum pressure that exists in the system so you can disconnect the vacuum line and separate the master cylinder from the booster. To do this, pump your brake pedal a few times with the engine off. Once the pedal firms up, you’re good to go.
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wiping down master cylinder

Step 2: Locate and Remove the Old Master Cylinder

Locate your master cylinder. Now is a good time to wipe down the area before opening up the brake system.
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disconnecting electrical lead from master cylinder

Remove the electrical connector leading to the brake fluid reservoir. This can be done by hand.
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removing clip from electrical connector on master cylinder

Remove the clip that secures the connector and push the tab to release it.
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unscrewing master cylinder mounting nuts

Unscrew the mounting nuts with your ratchet and socket and use the proper brake line flare wrench to disconnect the brake lines.

Do It Right: Put some shop towels in place and hold your fingers over the brake lines as you remove the master cylinder from the vehicle to make sure no brake fluid drips on anything.

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removing brake master cylinder from car

Leave the cap on and remove the master cylinder from the booster. There’s no need to remove the fluid.
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setting master cylinder in drain pan

Once you’ve removed the master cylinder, set it in your drain pan to allow any remaining fluid to drain. Keep in mind that a brake fluid spill will damage painted surfaces, so if this happens, flush it with water immediately.

Do It Right: If your vehicle has antilock brakes, you’ll also want to disconnect the brake line from the hydraulic control unit, or HCU, and remove this component so it’s out of the way.

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finger pointing out seal at end of master cylinder

Step 3: Bench Bleed the New Master Cylinder

Inspect the seal at the end of the master cylinder. There should be no sign of brake fluid leakage. If you see fluid, there is definitely a leak. If a leaky master cylinder isn’t replaced, it can also do damage to the brake booster.
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cleaning mounting surface for brake master cylinder replacement

Clean the mounting surface on the brake booster thoroughly with a shop towel and brake cleaner before attaching the master cylinder.

Do It Right: Again, be sure not to get brake cleaner on any painted surfaces.

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clamping down master cylinder in vise grip

When you install a new master cylinder, it will need to be bench bled. To do this, clamp the master cylinder securely by its mounting flange in a bench vice. If there are plugs in the outlet ports, they can be discarded.

Do It Right: Place a drain pan underneath.

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filling master cylinder reservoir with new brake fluid

Fill the reservoir with new brake fluid.
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installing threaded plugs on brake master cylinder replacement

Once fluid starts to drip from the outlet ports, install the threaded plugs.
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depressing master cylinder

Use a blunt tool to depress the master cylinder piston in three-quarters of an inch to one inch strokes, waiting ten to fifteen seconds between each stroke to keep air out of the fluid. Continue this until the piston can’t be depressed more than one-eighth of an inch, and no air bubbles are visible in the reservoir. Remove the master cylinder from the vice.
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removing threaded plug on brake master cylinder

Step 4: Install the New Master Cylinder

Now, we’ll install the hydraulic lines from the old master cylinder on the HCU. Start by removing one threaded plug at a time to connect the brake lines to the new master cylinder.
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positioning brake master cylinder in vehicle

Once that’s done, reposition the master cylinder and use the nuts to reattach it.
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reconnecting vacuum hose, HCU, and brake lines to master cylinder

Reconnect the vacuum hose, HCU, and brake lines.
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reinstalling electrical connector to master cylinder

Reinstall the electrical connector leading to the fluid reservoir.
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Connecting negative car battery terminal

Reconnect your negative battery terminal.

Do It Right: If you had to disconnect your brake lines, you’ll need to bleed the brake system.

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Now that everything is back together, start your engine and make sure your brake pedal pressure has been corrected by testing your brakes in the driveway before getting on the road.
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Additional Resources

bleeding brakes

How to Bleed Brakes
It’s possible for air to get into brake lines, and the result is spongy braking. Brake bleeding is necessary to remove the air from brake lines.
how to replace power brake booster in vehicle

How to Replace Power Brake Booster
If you’re noticing that it takes extra pressure on your brake pedal to slow down, you might be dealing with a failing booster.
differential fluid draining

How to Change Rear Differential Fluid
Changing your rear differential fluid is fairly straightforward on most vehicles and is an important maintenance item.

The automotive repair work depicted in this series is performed under the direction of the manufacturer of the product featured. Prior to undertaking any of the demonstrated repairs on your vehicle, we suggest you consult with a certified mechanic or another professional who can adequately advise you of the proper repair or remedy required. O’Reilly Automotive Stores, Inc. and its affiliates (“O’Reilly”) disclaims any responsibility for injury or damage resulting from a viewer’s attempt to recreate the repairs shown in this series.