A master cylinder is a critical component in a vehicle’s hydraulic brake system. It is responsible for pushing brake fluid through the lines to the wheel cylinders when the brake pedal is depressed. If a master cylinder fails, it can lead to a loss of braking power and create a very unsafe condition.
What is the role of the master cylinder?
The master cylinder converts the mechanical force from the brake pedal into hydraulic pressure. When the driver presses on the brake pedal, it pushes on the piston in the master cylinder, forcing brake fluid out under pressure. This pressurized fluid is then sent through the brake lines and hoses to the wheel cylinders at each wheel. The wheel cylinders contain pistons that are pushed outward by the brake fluid, in turn pushing the brake pads against the rotor to slow the vehicle.
The master cylinder is divided into two sections, each with its own piston and fluid reservoir. On most modern vehicles, there is one circuit for the front brakes and a separate circuit for the rear. This dual-circuit setup provides a safety advantage – if one circuit fails, the other can still work to stop the vehicle. However, brake performance will be diminished with only one circuit operational.
Common causes of master cylinder failure
There are several issues that can cause a master cylinder to stop functioning properly or fail completely:
- Worn-out seals – The master cylinder contains rubber seals that slide against the cylinder walls as the piston moves back and forth. Over time, these can become worn and develop leaks. Brake fluid will then bypass the piston, leading to a loss of pressure.
- Contaminated fluid – The brake fluid must be clean for the master cylinder to work correctly. If it becomes contaminated with dirt, water or other debris, it can damage the internal seals and cause them to leak.
- Air in the lines – Air trapped in the brake lines or cylinders will compress when pressure is applied, resulting in a spongy brake pedal feel. This indicates there is a leak somewhere in the hydraulic system.
- Corrosion – The master cylinder is made from metals that can corrode over time when exposed to road salt, humidity and some brake fluids. This can cause the cylinder bore and piston to seize up.
- Overheating – Excessively hard or repeated brake applications can overheat the brake fluid and cause it to boil. The expanding gases form bubbles which are compressible, leading to a loss of pressure.
Let’s take a more detailed look at some of the most common causes of master cylinder failures.
Worn seals
The seals inside the master cylinder are critical to its proper functioning. They maintain pressure and prevent fluid from leaking past the pistons. There are two main types of seals:
- Primary cup seals – These seals are located at the very tip of each piston. They slide directly against the cylinder walls and are responsible for generating hydraulic pressure. A damaged primary cup will cause the master cylinder to leak fluid internally.
- Secondary wiper seals – The wiper seals fit behind the primary cups and scrape along the cylinder walls to keep contaminants out of the fluid reservoir. If the wipers fail, dirt and debris can get pulled into the cylinder and damage the other seals.
The constant friction as the seals move back and forth during brake application eventually causes them to wear and lose their tight fit. Tiny grooves in the cylinder wall can also increase leakage past the seals. Once leakage begins, brake fluid will bypass the pistons and reduce the amount of pressure supplied to the brake lines. The brake pedal will feel soft and have excessive travel as more force is required to move the worn seals.
Seal wear is inevitable over time, so seals should be periodically inspected and replaced if found to be worn, cracked or swollen. The recommended interval is typically every 3 years or 36,000 miles.
Contaminated brake fluid
Clean, fresh brake fluid is essential for proper master cylinder operation. Contaminants like water, debris and minerals in the fluid can all lead to seal damage and failure:
- Water – Brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. The water lowers the fluid’s boiling point, potentially causing vapor lock in the system. It also promotes corrosion and pitting of internal cylinder surfaces.
- Debris – Dirt, sludge and other contaminants act as abrasive materials inside the cylinder. They scrape against the precision-fit seals and eventually wear grooves into the cylinder walls, leading to leaks.
- Mineral deposits – Heated, worn brake fluid can deposit a layer mineral scales and sludge inside the cylinder. This buildup scratches seals during operation and provides an uneven surface for them to slide against.
Preventing contamination involves periodically flushing the brake system and replacing the fluid according to the vehicle manufacturer’s recommendations, usually every 2-3 years. Many mechanics also recommend a complete brake system flush when installing a new master cylinder to remove any contaminants.
Air and vapor in brake lines
Whenever air or vapor bubbles are present in the brake hydraulic system, braking performance will suffer. Air is highly compressible, so when force is applied at the master cylinder, the air bubbles simply compress rather than transmitting the pressure through the fluid as designed. The result is a brake pedal that feels soft or spongy.
Air can make its way into the brake lines and master cylinder in several ways:
- A leak somewhere in the system is allowing air to be sucked in. Common leak points include brake hoses, calipers, wheel cylinders and the master cylinder seals.
- The fluid reservoir level falls too low, drawing air down into the master cylinder.
- After a brake component replacement, air gets trapped in the lines during the bleeding process.
- Vapor bubbles form when brake fluid overheats and boils due to excessive braking.
The solution is to identify and repair any leaks, maintain the proper fluid level, and thoroughly bleed the brake system to remove all air after component installations. Hard brake applications should be avoided when possible to prevent boil-over.
Corrosion damage
The master cylinder contains many precision parts made from steel and aluminum that can corrode when exposed to moisture. Road salt, winter conditions, humid climates and acidic brake fluid are all sources of corrosion issues.
The cylinder bore where the piston slides can become pitted and uneven. This wear causes seal damage and fluid leakage past the pistons. The pistons themselves may also seize up if corrosion builds up excessively.
Using the correct type of brake fluid for conditions can help minimize corrosion. For example, DOT 5 silicone fluid can resist corrosion better than glycol-based DOT 3 and 4 fluids. Periodically cleaning the master cylinder externally can also help prevent salt and dirt buildup. Replacing worn parts at regular intervals reduces the risk of master cylinder failure from corrosion damage.
Overheating
Excessively hard braking on long, steep downhill grades can overheat brake fluid, causing it to boil in the system. The hotter the fluid gets, the lower its boiling point becomes. Temperatures above 400°F can turn brake fluid into compressible vapor.
When overheated fluid starts to boil, it forms bubbles that are compressible. These vapor bubbles reduce braking power and cause the pedal to feel soft and spongy. Dangerously long stopping distances can result from overheated fluid.
Boiled fluid also accelerates corrosion and deposit buildup in the master and wheel cylinders. This wear and contamination leads to permanent damage over time.
To help prevent overheating, avoid riding the brakes excessively on long downhill stretches. Downshift the transmission to aid in slowing the vehicle and provide time for the brakes to cool between applications.
High-performance brake fluid with a higher boiling point, upgraded brake pads, and improved cooling ducts can help minimize overheating issues in high-demand use.
Symptoms of master cylinder failure
Some of the most common signs of a faulty master cylinder include:
- Low or spongy brake pedal
- Brake pedal slowly sinks to the floor when held down
- Brake warning light illuminated
- Difficulty stopping the vehicle
- Brake fluid leaking around the master cylinder
- Loose or broken mounting of master cylinder
Low or spongy pedal feel are signs that air, vapor or leaks are present in the hydraulic system, reducing pressure supply from the master cylinder. Pedal travel that continues to increase while braking indicates a leak. Any visible fluid leaks near the master cylinder seals needs immediate attention to prevent complete failure.
If the brake warning light comes on, it means there is likely an issue with the hydraulic system or fluid level. Similarly, increased stopping distances or difficulty slowing the vehicle points to low hydraulic pressure in the system.
Finally, a master cylinder that shows signs of physical damage or loose mounting could be allowing air into the brake lines. Any of these symptoms indicate master cylinder service is required to prevent potentially hazardous brake failure.
Testing for master cylinder failure
If you suspect your master cylinder may be having issues, there are a couple simple checks you can perform:
- Visual inspection – Look for signs of external leaks near the master cylinder. Check for loose mounting bolts or damage to the cylinder housing. Replace any visibly damaged units.
- Brake pedal check – Press the pedal with the engine off and watch for sinking or spongy feel. Have an assistant also check for brake light illumination with pedal pressed. Sinking pedal indicates low pressure supply.
- Brake fluid check – Look at fluid color and level in the reservoir. Dark or murky fluid needs flushing. Low level allows air ingestion into system. Proper level is within 1/4 inch of reservoir top.
- Leak-down test – Pump brakes to high pressure, then hold firm pressure for a minute. If pedal sinks or can be pushed further, it signifies leaks. Repeat test with engine running to check operation under vacuum.
A mechanic can perform additional diagnostic checks of the full hydraulic system. But these basic master cylinder tests can identify external leaks, seal failures, air presence, contamination, and mounting issues.
Replacing a faulty master cylinder
If testing confirms the master cylinder needs to be replaced, follow these steps for a safe and complete repair:
- Purchase an exact OEM replacement or quality aftermarket master cylinder for your specific vehicle make and model. Do not attempt to install a universal fit part.
- Clean the master cylinder reservoir and surrounding brake lines thoroughly before installation. Flush the brake system if fluid is old or contaminated.
- Check for any bent, cracked or seized brake lines and replace as needed. Use new brake hoses.
- Follow torque specifications when tightening all brake fittings and mounting hardware.
- Bench bleed the new master cylinder before installing to prime it and expel any trapped air.
- Fill the reservoir with correct fluid type and maintain proper level throughout job.
- Fully bleed the brake hydraulic system using a pressure bleeder or manual bleeder until fresh fluid flows from all bleed points.
- Check for leaks and confirm firm brake pedal feel before driving.
Replacing just the master cylinder may temporarily solve an underlying issue, but the root cause still needs to be determined in order to prevent another failure. Thoroughly flushing the system, checking for leaks and bleeding out all air after installation helps ensure the best longevity of your new master cylinder.
Preventing master cylinder failure
Regular maintenance and driving habits can help extend the life of your master cylinder and brake system components:
- Flush brake fluid and replace every 2-3 years to prevent contamination.
- Check fluid reservoir level monthly and top off as needed.
- Visually inspect brake lines, hoses and seals for leakage.
- Do not overly rely on brakes going downhill. Use downshifting to aid deceleration.
- Avoid hard braking unless necessary to minimize heat buildup.
- Address any spongy or low pedal feel immediately to prevent further damage.
- Fix any hydraulic leaks promptly before air intake develops.
Periodically servicing the brake fluid, pads, rotors, hoses and seals can help extend the life of your master cylinder. Driving habits also matter – don’t ride the brakes excessively and allow them time to cool when braking hard or frequently. Address any symptoms of low pressure or leaks right away. Your master cylinder will reward you with reliable braking performance.
Conclusion
The master cylinder is the heart of the hydraulic brake system, supplying the pressurized fluid needed for safe stopping. Seal wear, contamination, air ingestion, overheating and corrosion are the major causes of master cylinder failure over time and mileage. Stay ahead of problems by monitoring pedal feel, performing routine brake fluid maintenance, and avoiding excessive brake heat. Replace the master cylinder at the first sign of leaks or pressure loss. With proper component condition and bled brake lines, your master cylinder should deliver miles of reliable braking performance.
