Any vehicle is only as safe as its brakes. Yes, fast acceleration and hugging the pavement as you corner is fun – but failing to stop on time is catastrophic. Paying attention to your brakes will keep you from wrapping your family haulier around a street light or your track day car around the barriers.
While most people will diligently check their brake levels, replace the brake pads on time and even swap their brake rotors once they start wobbling or thinning, very few give the very important master cylinder second thought.
What is the Master Cylinder?
The master cylinder is a crucial part of the entire system. It distributes braking force from the foot pedal and the brake booster to the brake lines and, therefore, to all the brakes in your vehicle.
When you push in the brake pedal
- The piston on the brake pedal will push a piston on the master cylinder
- The plunger on the master cylinder then pushes fluid from the primary and secondary brake fluid reservoir, transmitting hydraulic pressure to slave cylinders on each wheel
- The slave cylinders on each wheel then push brake pads or brake shoes against the brake rotor or drum, slowing down the car
This is a massive simplification since we have overlooked the brake booster feeding additional hydraulic pressure to the braking system.
You rarely have to think about the master cylinder since, unlike slave cylinders, it is well-protected in a confined space under the hood. This, combined with the fact that it is designed to last your car’s lifetime, means very few car owners ever have to service or replace their master cylinders.
However, you might be among the unlucky few if:
- Your vehicle has a know fault with the system
- You drive a very high mileage vehicle that is way beyond the expected lifespan
- You are, well, just unlucky
Signs that You Should Test Your Brake Master Cylinder
You never have to check your brake master cylinder unless you suspect it is faulty. Knowing the signs and symptoms to check out for will help you catch the problem before the master cylinder fails catastrophically.
Even though the master cylinder is stupidly simple and hard to fail, it could develop some hard-to-detect internal leaks.
- Leaks are not as obvious as it will leak into the vacuum booster through the rear seal or leak back into the brake fluid reservoir
- Contaminated brake fluid that makes the master cylinder stick and stutter
Your Brake Fluid Levels Drop Drastically
The first sign of a problem in your brake system is a drastic dip in brake fluid levels within the reservoir. While it is normal for the levels to drop as your brake fluids wear out, it should never go below the minimum line.
The fluid could leak into the booster system, making it hard to see or between the piston from the brake pedal and the master cylinder into a spot that is hard to notice.
Inspect your entire braking system to identify a leak point whenever you notice an unacceptable drop in your brake fluid levels.
A Check Engine Light or ABS Warning
Most modern vehicles with ABS or some brake-assisted stability control have sensors on the master cylinder and other parts of the braking system to monitor brake fluid pressure. The sensor’s main function is modulating automatic braking when the ABS kicks in or the vehicle tries to regain stability using automated functions.
By monitoring the fluid’s pressure, the system can estimate how much braking force it expects. If the sensor detects too low pressures from the master cylinder when it expects more, it will throw an error code in the ABS and stability control module.
The error could mean your master cylinder is faulty or something else is the matter with your braking system.
Unexpected Brake Pedal Behavior
After driving vehicles for a while, you get to understand the normal brake pedal behavior. The gradualness, the subtle resistance and expected braking force. With time, this normal behavior is coded into your expectations when driving.
You should be worried any time the braking system behaves otherwise. Some of the top behaviors that hint on the master cylinder’s failure to distribute braking force adequately include:
- Spongy feeling pedal
- Low braking force send to the wheels making your vehicle take longer to stop
- A brake pedal that slowly sinks to the ground when pressed without the expected gradual increased resistance
- Erratic changes in brake pedal travel and resistance
Note that these symptoms might also hint at other issues within your brake system. Start with easier to troubleshooting and fix issues like wornout calipers, leaking brake lines or slave cylinders, and damaged brake booster before finally settling on the master brake cylinder.
Black Pieces of Contaminants in the Brake Fluid
Your braking system doesn’t have a filter. Any solids or alien particles will float in the fluid and you can see them in the reservoir if you are keen.
In case one of the rubber seals on your master cylinder go bad, they will end up in the brake fluid. You will either see small bits of them or they will disintegrate into finer bits giving the brake fluid a dark brown or black color.
If you have reason to suspect your master brake cylinder has given up the ghost, it is time to test it and confirm your fears or rule them out and look into different parts of the brake system to fix the problem you noted.
How to Test the Master Cylinder While It’s Still in the Car
Ensure that the master cylinder and brake system are properly bled before attempting this.
- Pump the brake pedal three to five times and hold
- When you hold, the brake pedal should remain firm and not spongy
- If, during the pumping, the brake pedal isn’t pushing back and springing back to the neutral position, you could be dealing with a stuck cylinder
- A spongy feel could mean the system is not well bled
- If the pedal slowly drops to the metal, then the master cylinder is leaking fluid and pressure through one of its seals
Bench Testing Your Brake Master Cylinder
Bench testing is the best when you replace the cylinder or already have it out of the vehicle. It is similar to the tests you do before installing a new or remanufactured unit.
- Start by bleeding the master cylinder to remove any air from it
- Remove the bleeding plumbing while holding the master cylinder under brake fluid
- Block the cylinder’s ports with bolts taking care not to strip them
- With a screwdriver, press and hold onto the plunger on the rear side of the master cylinder
- The plunger should be very firm and impossible to move past a few millimetres
- If you can push the plunger more than this, there is a leak in one or more of the internal seals
Remember that if your master cylinder passes the tests or you end up replacing it, you still have to bleed it properly and fix any other worn-out parts in the brake system before closing down the project.