What Happens If You Overfill Brake Fluid? 10 Issues Arise

Brake fluid gives you hydraulic power to brake with foot pressure. As it flows through brake lines and engages pistons in brake calipers, it enables your car to stop. Remember that insufficient brake fluid leads to brake failure, while too much can lead to life-threatening problems. Therefore, regularly checking and changing the brake fluid ensures safe and stable braking.

With this information handy, one might ask: What happens if you overfill brake fluid? It’s simple: overfilling the brake fluid creates additional pressure in the brake lines. As a result, component failure or leaks can happen. Moreover, moisture contamination can reduce the fluid’s boiling point, leading to accelerated wear, leaks, sensor damage, freezing in cold temperatures, less effectiveness, and braking system failure.

You may also read What can I use Instead of Brake Fluid?

In this article, we’ll discuss how overfilled brake fluid impacts your vehicle’s braking system and what you can do if brake fluid level is too high.  

What Happens If You Overfill Brake Fluid? 10 Major Issues arise

Issue arise due to overfill brake fluid

One of the most common question is: What happens if you overfill your brake fluid? So, I am here to answer this question. Ten major issues arise if you overfill the brake fluid. Now, we discuss these issues one-by-one.

1. Wear and tear to Brake Components

Adding brake fluid above the recommended level increases hydraulic pressure, forcing the brake pads to push harder against the rotor or drum. This causes the friction material to wear down rapidly, shortening brake pads’ service lives.

Additionally, it stresses the rubber seals and o-rings in the brake system, leading them to fail and leak more frequently. As a result of the increased pressure, pistons in calipers and wheel cylinders are forced out further, resulting in brake pads and rotors to wear unevenly.

2. Oxidation of the Brake Fluid

A small amount of moisture naturally accumulates in brake fluid over time, but excess fluid in the reservoir exposes more surface area to air, thereby absorbing moisture faster. As a result, the brake fluid oxidizes, breaks down, and reduces its boiling point significantly due to moisture contamination. 

Moreover, this boiling point drop causes vapor lock in hard braking conditions, reducing hydraulic braking force transmission efficiency.

3. Brake Fluid Level Sensor Failure

Modern brake master cylinders contain float-style electronic sensors that measure fluid levels. Consider what happens if you overfill brake fluid; these sensors get stuck in the upward position, unable to drop down as fluid is displaced.  

Therefore, inaccurate level readings may result in dash warning lights or a check engine light, leading owners to believe something else is wrong.

4. Braking Performance Reduced

In a hydraulic system with too much brake fluid, tiny air bubbles collect and circulate, causing the brakes to produce insufficient clamping force, which increases stopping distances. Since full hydraulic pressure can’t reach the wheels or calipers, they feel soft and spongy.

5. Leaky Reservoir

Overfilled brake fluid can increase internal pressure on the plastic reservoir, leading to cracks at weak seams and drips from the reservoir. Occasionally, high pressure can push past the reservoir cap seals and into the brake lines, carrying contaminants that may have overflowed from the reservoir and into the brake lines. 

As a result, the brake fluid reservoir breaks down much faster.

6. Brake Dragging and Overheating

There is no way to completely retract the pistons into the bores when the brake fluid is too much in the caliper and wheel cylinders. Therefore, drag forces the brake pads to rub lightly against the spinning rotor or drum. 

Additionally, friction wears out the brake pads faster and overheats the rotor or drum. This can lead to brake fade and uneven rotor wear.

7. Frozen Brake Lines

When temperatures drop below freezing, the moisture absorbed by overfilled brake fluid can freeze inside the brake lines and components. Consequently, what happens if you overfill brake fluid is the brake pedal falls straight down without stopping power due to ice crystals forming inside the brake fluid.

8. Damage to Rubber Brake Components

Seals, o-rings, gaskets, and rubber brake hoses are not meant to be constantly exposed to moisture-contaminated brake fluid. Hence, when overfilled, the extra fluid saturation causes them to dry, swell, soften, and crack. The result is leaks at seals and splits in brake hoses, requiring early replacement of all rubber parts exposed to the fluid.

9. Brake System Corrosion

Under conditions where brake components like calipers, wheel cylinders, and lines are constantly exposed to overfilled brake fluid, they tend to rust and pit. Often, corrosion occurs inside piston bores, line walls, and other mating surfaces, and galvanic corrosion accelerates when brake fluid is wet.

10. Brake Bleeding Trouble

The removal of all air bubbles from the hydraulic system is much more difficult when there is excess fluid overflow in the system. Therefore, incomplete bleeding causes soft pedals and reduced stopping power because more air is trapped and drawn back in. 

Accordingly, for the pedal to feel firm and brake effectively, it must be properly bled.

Best Way to Maintain Proper Brake Fluid Levels

When checking and refilling your brake fluid, follow these instructions to ensure the safety of your vehicle.

Fluid Reservoirs Position

A brake fluid reservoir is usually located under the hood near the firewall or brake master cylinder, near the engine bay. Moreover, your vehicle typically has two brake fluid reservoirs – one at the front and one at the rear. Thus, read your owner’s manual to know where they are, so you can keep your fluid level up.

Check the Brake Fluid Level

Check the brake fluid inside the reservoirs with your vehicle parked on a level surface. It should be between the “Min” and “Max” lines marked on the tank. 

Observe: If it is below the minimum line, that indicates that some brake fluid has leaked out or been consumed and needs to be refilled. Alternatively, if the level is above the maximum line, too much fluid has been added, which can cause major problems.

Use the Proper Brake Fluid Type

You need to use brake fluid based on the manufacturer and model of your vehicle. Consequently, you should refer to the vehicle’s owner’s manual to determine what brake fluid you need. 

Typically, common types are DOT 3, DOT 4, and DOT 5.1, but some vehicles require specific types. Note that adding the wrong fluid type can severely damage your braking system components. 

Prevent Contamination of the Fluid

If you plan to refill the brake fluid reservoir, take steps to prevent contaminants from entering. Do not use old brake fluid from previously opened bottles because it may contain moisture contamination. Instead, use new, unopened bottles of brake fluid. 

Also, be careful not to use fluid stored for an extended period, as unopened bottles still have a limited shelf life.

How to Fix Overfilled Brake Fluid Reservoirs  

You might have an overfilled reservoir if your brakes require repair. The following steps will guide you through fixing an overfilled reservoir.

Step 1: Check the brake fluid reservoir

You will usually find the brake fluid reservoir underneath the hood near the firewall. Then, verify the fluid level on the side of the reservoir. If it is above the “Max” fill line, determine approximately how much extra fluid is present. Hence, you know how much must be removed to get it back to the proper level.

Step 2: Extract Excess Fluid

With the vehicle turned off and cooled down, use a turkey baster or brake fluid siphon to carefully suck out the excess fluid above the “Max” line. However, work slowly and steadily to extract the overflow fluid until it returns to the normal level. 

Note: Keep fluids in a container for disposal; do not spill them.

Step 3: Check Brakes for Leaks

Once the reservoir is back to its proper level, check all brake components for leaks caused by overfilled fluid. Specifically, look along brake lines, calipers, wheel cylinders, and rubber hoses for signs like wetness, swelling, soft spots, or other damage. Identifying and repairing leaks will prevent further fluid loss from what happens if you overfill brake fluid.

Step 4: Examine brake pads and rotors

In the meantime, inspect brake pads and rotors to see if excess fluid pressure has caused accelerated wear or damage. Accordingly, if excess wear is found on the pads or rotors, it is recommended that the pads and rotors be replaced to achieve proper braking.

Step 5: Bleed the brakes

Suppose the brake pedal feels soft or spongy after pumping it several times. In that case, the brake fluid has overfilled and introduced air bubbles into the lines. 

Without delay, bleed the brakes to release any trapped air and firm up the pedal. Your repair manual will provide instructions on how to do this.

Step 6: Check brakes carefully

Before going on a routine drive, road test the vehicle in a parking lot or safe area away from traffic. Also, ensure the brakes are not pulling, dragging, grinding, or exhibiting reduced stopping power. Still, identifying any issues requires further troubleshooting and repair.

Bottom Line

Maintaining the safety and effectiveness of your vehicle’s brake system is key to a secure driving experience. Moreover, frequent brake fluid changes are preventable by understanding what happens if you overfill brake fluid and taking preventive measures, such as checking your brake fluid regularly.

Similarly, proactive maintenance prevents potential technical issues from escalating, resulting in significant cost savings. Keep an eye out and promptly address any signs of fluid change or leakage to preserve the integrity of your brake system and enhance road safety.

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