Brake Fluid Leaks: Types, Signs, Effects & More
What is Brake Fluid?
Brake fluid is a high boiling point hydraulic fluid that is used to transfer pressure to the braking system in vehicles. It works by transmitting the force applied on the brake pedal to the brake pads or shoes in order to stop the wheels from turning.
Brake fluid is different than brake oil. Brake oil is used in older mechanical brake systems to lubricate the brake components. Modern hydraulic brake systems use brake fluid which does not have lubricating properties.
There are different types or “grades” of brake fluid as defined by the U.S. Department of Transportation (DOT). These include:
- DOT 3 brake fluid – This has a minimum dry boiling point of 401°F (205°C). It can be used in disc, drum and anti-lock braking systems in passenger vehicles.
- DOT 4 brake fluid – This has a higher minimum boiling point of 446°F (230°C). It offers better performance under high temperatures and can be used in disc and anti-lock braking systems.
- DOT 5 brake fluid – This is silicone-based and does not absorb water over time like DOT 3 and DOT 4 fluids. It has an even higher minimum boiling point of 500°F (260°C).
Always check your vehicle’s owner’s manual for the recommended type of brake fluid. Using the proper brake fluid designed for your specific braking system is critical for safe operation.
Causes of Brake Fluid Leaks
There are several common causes of brake fluid leaks in vehicles:
Worn Brake Hoses/Lines
The brake hoses and lines transport brake fluid through the system. Over time, the rubber hoses can crack, split or bulge, allowing fluid to leak out. The steel brake lines can also rust and corrode from the inside, eventually causing a hole or crack that leaks fluid.
Failed Wheel Cylinder
Wheel cylinders are hydraulic actuators that apply force to the brake pads when you press the brake pedal. The cylinders contain pistons that can wear out or fail, leading to fluid leaks. A failed wheel cylinder will usually cause leaking from the wheel area.
Leaking Master Cylinder
The master cylinder pressurizes the brake fluid when you depress the brake pedal. Master cylinder seals can wear out or sustain damage, resulting in external leaking. An internal leak in the master cylinder will allow fluid to travel back to the reservoir.
Loose/Corroded Fittings
The various brake lines are connected by fittings and joints. If these fittings become loose due to vibration, or corroded over time, they can allow brake fluid to slowly seep out. It’s important to inspect all fittings and connections when diagnosing a brake fluid leak.
Signs of a Brake Fluid Leak
There are several signs that may indicate your vehicle has a brake fluid leak. Being aware of these signs and addressing any leaks promptly can help avoid potentially dangerous situations.
The most obvious sign is visible brake fluid under your car. Brake fluid is typically a light amber or yellowish color. If you notice any fluid on the ground under your car that matches this description, it likely points to a brake fluid leak.
Another sign is if the brake fluid reservoir under the hood is low. The reservoir houses brake fluid for the entire system. A drop in fluid level indicates a leak somewhere in the system. Be sure to only check the reservoir when the engine is cold, as heat causes the fluid to expand.
You may also notice a “spongy” brake pedal, which means you have to push it farther to get the same stopping power. This spongy sensation indicates air in the brake lines caused by a fluid leak. It’s an extremely dangerous driving condition, as you can lose braking capability.
Finally, the brake warning light illuminating on the dash could signal low brake fluid and a leak. Most brake systems have a float that drops with fluid level, triggering the light. Don’t ignore this warning light under any circumstances.
Dangers of Driving with a Brake Fluid Leak
Driving with a brake fluid leak is extremely dangerous and should never be done. A brake fluid leak will cause a loss of brake pressure, leading to significantly longer stopping distances. In severe cases, it can even lead to complete brake failure where the brakes are unable to stop the vehicle at all.
The dangers include:
- Longer stopping distance – With less brake fluid, the brakes will have reduced hydraulic pressure. This means it will take longer for them to slow and stop the vehicle. Braking distances can be dramatically increased, taking over twice as long to stop.
- Complete brake failure – If the leak is serious enough, it may lead to a complete loss of brake fluid resulting in zero braking ability. The pedal will go straight to the floor with no stopping power at all.
- High accident risk – The greatly extended stopping distance combined with potential complete brake failure makes an accident extremely likely. A brake failure at highway speeds or in heavy traffic can easily result in collisions and serious injuries.
Driving any vehicle with a brake fluid leak is putting yourself and others in grave danger and should never be attempted. Have any leak repaired immediately before operating the vehicle.
Testing for a Brake Fluid Leak
There are a few simple tests you can perform to check for a brake fluid leak:
Visual Inspection
Pop open the hood and inspect the brake master cylinder reservoir. If the fluid level is low, then you likely have a leak. The reservoir is usually located near the firewall on the driver’s side of the engine bay. The fluid should be filled to the “Full” line – if it’s below that, fluid is likely leaking out somewhere.
Also visually inspect brake lines, calipers, the ABS modulator, and wheel cylinders for any signs of wetness or fluid spray. Leaks often originate at the connections between metal brake lines. The leak may be small, so look closely.
Pressure Test
A more thorough test is to have a professional mechanic use a pressure tester. This tool pressurizes the brake system to locate even small leaks that may not be visible. The pressure tester attaches in place of the brake fluid reservoir.
Pedal Test
With the engine off, pump the brake pedal several times to deplete the brake booster’s vacuum reserve. Then hold down the pedal – if it slowly sinks down farther, that indicates a leak in the hydraulics. This brake pedal test can help pinpoint the location of the leak before a full pressure test.
Performing these simple checks can help identify if and where brake fluid may be leaking from your vehicle. Finding the leak early and getting it repaired prevents bigger problems down the road.
Fixing a Brake Fluid Leak
If you have identified a brake fluid leak in your vehicle, it’s important to repair it promptly to avoid potentially dangerous brake failure. Here are the steps for fixing a brake fluid leak:
Identify the Source of the Leak: Visually inspect the brake lines and components like calipers, wheel cylinders, and the master cylinder for any sign of wetness or dripping fluid. This can help pinpoint the source of the leak so the worn or damaged part can be replaced.
Replace Worn Parts: If a brake line is corroded or cracked, it will need to be replaced. Other common parts that wear out and cause leaks are wheel cylinder seals, caliper seals, and master cylinder seals. Replacing the leaky seal or component will stop the leak.
Flush the Brake Fluid: After fixing the actual leak, the old contaminated brake fluid will need to be flushed from the system. Use a brake bleeder tool to siphon out old fluid and replace it with fresh, clean brake fluid. Make sure to bleed the brakes at all four wheels to completely purge the contaminated fluid.
Properly diagnosing and repairing a brake fluid leak is crucial for safe brake operation. Allowing leaks or contaminated fluid to remain in the system can lead to brake failure and greatly increase the risk of an accident. Fix any identified leaks right away before driving the vehicle again.
Preventing Brake Fluid Leaks
There are a few key things you can do to help prevent brake fluid leaks from occurring:
Regular Brake Inspections
Have your brake system inspected by a professional mechanic on a regular basis. Many mechanics recommend having your brake system looked at every time you have your oil changed or get regular maintenance done on your vehicle. During an inspection, the mechanic will be able to spot any wear and tear on brake lines, calipers, wheel cylinders or master cylinder and proactively replace parts before leaks occur.
Replacing Worn Parts
If any part of your brake system is identified as being excessively worn down or corroded, have it replaced right away. Waiting too long increases the likelihood of cracks and holes developing which can lead to fluid leaks. Replacing worn parts promptly reduces the chances of a leak.
Proper Fluid Maintenance
Make sure to have your brake fluid flushed and replaced per the recommendations in your owner’s manual, usually every 2 years. Over time brake fluid absorbs moisture which lowers its boiling point and increases the chances of brake failure. Keeping fresh fluid in the system minimizes corrosion.
Cost to Repair a Brake Fluid Leak
The cost to repair a brake fluid leak can vary greatly depending on the severity of the leak, parts needed, and labor time. Here are some of the factors that affect the repair cost:
Parts Cost
The parts needed to fix a brake fluid leak typically include new brake lines, calipers, wheel cylinders, or master cylinder. Brake lines can range from $15-$60 each, brake calipers are $100-$300, wheel cylinders around $50, and a master cylinder around $100. The more parts that need replacement, the higher the total parts cost.
Labor Time and Rate
A minor leak may take 1-2 hours of labor to fix by replacing a single brake line. More extensive leaks involving multiple components can take 3 hours or more, with labor rates often around $100 per hour. The exact repair time depends on the leak’s location and complexity.
Severity of the Leak
A small leak from one brake line may cost $200-$300 to fix in parts and labor. Moderately leaking calipers or cylinders may cost $400-$600. A major leak requiring replacement of multiple components and flushing all brake fluid could cost $800-$1,200 or more. The faster it leaks, the more parts affected, and the higher the overall repair bill.
Signs of Contaminated Brake Fluid
Brake fluid can become contaminated over time through the absorption of moisture from the air. This leads to a few telltale signs that indicate your brake fluid needs to be flushed and replaced:
Dark/Muddy Color: Fresh, uncontaminated brake fluid has a light amber color. As it absorbs moisture, the fluid will begin to take on a darker, muddier tone. This is a result of corrosion and oxidation occurring within the system due to water mixing with the brake fluid.
Lower Boiling Point: The boiling point of fresh brake fluid is typically around 500°F. However, as moisture builds up in the fluid, its boiling point drops significantly lower. This is dangerous because it increases the chances of brake fade or complete brake failure from fluid boiling under hard braking.
Compromised Braking Ability: With contaminated fluid, you may start to notice changes in braking performance, such as a soft or spongy pedal feel. Braking distances will become longer. This is all caused by moisture interfering with proper hydraulic pressure delivery throughout the brake system.
When to Change Brake Fluid
Brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. This moisture contamination lowers the boiling point of the brake fluid, potentially leading to vapor lock and brake failure. For this reason, brake fluid should be changed regularly.
Most vehicle manufacturers recommend flushing and replacing the brake fluid every 2 years or 24,000 miles as part of routine maintenance. This interval helps remove moisture before it can significantly contaminate the fluid. Some luxury brands like BMW recommend brake fluid changes even more frequently – every 1-2 years regardless of mileage.
Brake fluid may need to be changed more often if the vehicle sees heavy use or operates under extreme conditions. Frequent high-speed driving, towing heavy loads, and mountainous or aggressive driving can all introduce more heat and moisture into the brake system. For these severe duty vehicles, consider changing the brake fluid annually.
Lastly, check the brake fluid periodically for signs of contamination like discoloration and particles. If the fluid appears dirty, oxidized, or separated, have it flushed and replaced immediately. Don’t wait for the regular maintenance interval if moisture contamination is apparent.