The brake master cylinder is what generates pressure in the brake fluid when you press the brake pedal, and from there it is distributed to the calipers or wheel cylinders at the four wheels. The master cylinder is bolted to the brake booster, which is bolted to the firewall in the engine bay. Inside, the master cylinder has two pistons and seals in two separate hydraulic chambers, usually one for the rear brakes, and one for the front brakes. Each chamber has its own fluid outlet, so there are two brake lines fixed to the master cylinder, which send brake fluid under pressure to the two halves of the system. What happens most often is that age or wear will eventually cause a seal on one of the pistons to begin to fail, and then only half of the brake system will receive full pressure.
Replacing the master cylinder is usually fairly straightforward, except for the procedure to eliminate air from the system. This is important because air compresses, so any air at all in the brake system will give the brakes a soft or spongy feeling and reduce their effectiveness. In most cases, the master cylinder will be “bench bled” before installation, to eliminate air from the two internal cylinders, and then pressure bled out to each of the four wheel after installation. Ideally, the result is brakes that feel smooth, solid, and even, and a vehicle that stops predictably and in a straight line.
Cost of Brake Master Cylinder Replacement
On average for most vehicles, it costs about $275 to replace the master cylinder.
For some more specific estimates of the repair cost on some common vehicles, using $100 as a labor rate:
For a 2011 Ford Explorer with a 3.5 liter engine, the labor time to replace the master cylinder is 1.6 hours. A factory master cylinder costs about $135, and a Wagner part costs about $113. This makes the job about $295 using factory parts, or about $273 using aftermarket parts.
For a 2006 Honda Accord with a 2.4 liter engine, the labor time to replace the master cylinder is 1.6 hours. A factory part costs about $238, and a Centric part costs about $95. This makes the job about $398 using factory parts, or about $255 using OE parts.
For a 2008 Chevrolet Malibu with a 3.6 liter engine, the labor time to replace the master cylinder is 1.6 hours. A factory master cylinder costs about $152, or a Raybestoes part costs about $90. This makes the job about $312 using factory parts, or about $250 using aftermarket parts.
How to Tell if the Master Cylinder Might Need Replaced
The obvious one would be that the brake light on the dash might come on. There is a brake warning light and an ABS light; the brake warning comes on if there is an imbalance in the brake system, such as a leak at one corner or a failure in one half of the master cylinder could cause. The ABS light would be unrelated, being set if the module sees an electrical issue primarily.
If one of the two sides of the brake master cylinder isn’t sealing or delivering pressure, that will trip a sensor in the proportioning valve and turn the brake warning light on. The brake pedal might also feel “soft”, sinking under light pressure without engaging the brakes. Depending on how the brake system is organized this could also cause a pull to one side or the other, or cause the front or rear brakes to lock up more easily, the result of only two out of the four corners braking effectively.
Another symptom can be leaks. A master cylinder will usually fail internally without giving any outward signs, but sometimes the rear seal can leak. In some cases, this goes right into the brake booster (which ordinarily then would need to be replaced as well), or in many cases, it runs out between the master cylinder and the booster where they are bolted together. If the rear seal leaks, it can also allow air to be introduced into the system. If a vehicle’s brake system has been bled out well, then it later comes back with a spongy pedal needing bled again, it’s often a faulty rear seal at the master cylinder that’s the cause.
What to do if a Master Cylinder is Contaminated
The first thing that is looked at in most brake inspections is the condition of the fluid. There are a couple kinds of possible contamination. The first is water; brake fluid is hygroscopic, which means it absorbs water. The master typically has a small port for air to enter to displace fluid during use, so it is exposed to moisture from the air, and it absorbs moisture over time. Too much moisture in the brake fluid can cause corrosion inside the system, and it can reduce the boiling point of the brake fluid to a dangerous point. That’s one of the main reasons to do a brake fluid service.
But petroleum products are a much worse kind of contamination if they are introduced into the brake system. In the event that someone makes a mistake and tops off the brake master cylinder with power steering fluid or engine oil or something of that sort, the result is that the rubber in the master cylinder swells up and loses its ability to form a seal. That’s only the first part of the problem. Often the contaminants travel down through the system, damaging the brake hoses, then the calipers or wheel cylinders. Contaminated calipers will usually seize up, while wheel cylinders will leak. The basic recommendation from all manufacturers if a vehicle has brake fluid contaminated by a petroleum product is to replace everything in the brake system that has rubber parts exposed to the fluid.
One way to tell is to look in the master cylinder with a flashlight, and see if there are globules of fluid floating on top which are not mixing with the ordinary brake fluid. Another way is to draw off a sample and see if it is completely water-soluble – if it will absorb and mix with ordinary water.
If the brakes don’t feel right, if the pedal sinks or if the brakes lock up or pull, or if the brake warning light is on, it’s a good idea to have it checked right away.
The main thing is to regularly replace the brake fluid. Most manufacturers recommend a brake fluid flush every 30,000 to 50,000 miles to keep it clean and new, and to prevent problems that can be caused by moisture contamination.
Probably not a good idea. Replacing a master cylinder is easy enough, usually, but there are special procedures and tools involved in purging the air from a master cylinder before installation, and there are special tools and procedures to purge the air from the brake system after installation. Even with the right tools, headaches are not uncommon.