Brake fluid is a vital component of a vehicle's braking system, responsible for transferring the force from the brake pedal to the brake calipers or wheel cylinders. However, the presence of air in the brake fluid can significantly compromise the effectiveness of the braking system, leading to potential safety hazards. In this comprehensive guide, we will explore the causes of air in brake fluid, the symptoms to watch out for, and the proper steps to bleed the brakes and restore optimal braking performance.
Air can find its way into the brake fluid system through various avenues, leading to reduced braking performance and potential safety hazards. Let's explore the main culprits:
| Cause | Description |
|---|---|
| Brake Fluid Absorbing Moisture | Brake fluid is hygroscopic, meaning it absorbs moisture from the surrounding air over time. As the brake fluid becomes saturated with water, its boiling point decreases, causing air bubbles to form in the brake lines. |
| Leaks in the Brake System | Air can enter through leaks in various components, such as brake lines, calipers, wheel cylinders, or master cylinder seals. Even the smallest leak can act as an entry point for air. |
| Improper Brake Bleeding | If the brake bleeding process is not performed correctly after repairs or component replacements, air pockets can remain trapped in the brake lines. |
| Low Brake Fluid Level | When the brake fluid level drops too low, air can be drawn into the master cylinder reservoir and subsequently into the brake lines. |
Brake fluid is hygroscopic, meaning it has a tendency to absorb moisture from the surrounding air over time. As the brake fluid becomes saturated with water, its boiling point decreases, causing air bubbles to form in the brake lines. This phenomenon can occur even in a well-sealed system, as the fluid gradually absorbs moisture through microscopic pores in the components.
The rate at which brake fluid absorbs moisture depends on various factors, including the type of fluid, the age of the fluid, and the environmental conditions. Older brake fluid or fluid that has been exposed to high humidity or temperature fluctuations is more prone to moisture absorption.
Air can enter the brake system through leaks in various components, such as brake lines, calipers, wheel cylinders, or master cylinder seals. Even the smallest leak can act as an entry point for air, allowing it to be drawn into the system as the brake pedal is released.
These leaks can be caused by several factors:
Wear and Tear: Over time, the constant flexing and vibrations in the brake system can cause components to wear down, leading to leaks.
Corrosion: Exposure to moisture, road salt, and other environmental factors can cause corrosion in brake components, compromising their integrity and leading to leaks.
Physical Damage: Accidents, debris on the road, or improper maintenance can result in physical damage to brake lines, hoses, or other components, creating potential leak points.
Whenever brake repairs or component replacements are performed, it's crucial to properly bleed the system to remove any trapped air. The brake bleeding process involves forcing brake fluid through the lines and components to expel any air pockets.
If the bleeding process is not carried out correctly, air pockets can remain in the brake lines, compromising the system's efficiency. Improper bleeding can occur due to various reasons, such as:
Failing to follow the manufacturer's recommended bleeding sequence
Using incorrect bleeding techniques or equipment
Not ensuring that the brake fluid reservoir is topped up during the bleeding process
The brake fluid level in the master cylinder reservoir plays a crucial role in preventing air from entering the system. If the fluid level drops too low, air can be drawn into the system through the compensating ports in the master cylinder.
A low brake fluid level can occur due to several reasons:
Fluid leaks in the system
Worn brake pads or shoes, which consume more fluid as they wear down
Neglecting to top up the fluid level during routine maintenance
Identifying the presence of air in the brake fluid system is crucial to address the issue promptly. Here are the steps I typically follow during inspection and diagnosis:
Check the brake fluid level in the reservoir and top it up if it's low.
Carefully inspect all brake components, lines, and hoses for any signs of:
Leaks
Damage
Wear
Any of these issues could allow air to enter the system.
| Symptom | Indication |
|---|---|
| Spongy or soft brake pedal | Air in the system |
| Excessive brake pedal travel | Air in the system |
Press the brake pedal and observe its feel and travel distance.
If the pedal feels spongy or soft, or if it needs to be depressed further than usual to engage the brakes, it's a strong indication that air is present in the brake lines.
Another telltale sign of air in the brake system is inconsistent braking performance. If the brakes work fine sometimes but feel less effective at other times, it's likely due to air pockets moving around in the lines.
Once air in the brake fluid system has been identified, it's crucial to address the issue promptly to restore proper braking performance and ensure safety. Here are the typical repair procedures I follow:
The most common repair method is to bleed the brake system, which involves removing the trapped air from the brake lines.
Follow the manufacturer's recommended bleeding sequence, typically starting with the wheel farthest from the master cylinder.
Various bleeding methods can be employed:
Using a vacuum bleeder
Using a pressure bleeder
Gravity bleeding technique
During the bleeding process, it's essential to:
Ensure that the brake fluid reservoir is topped up to prevent introducing more air into the system.
Use the correct brake fluid type specified by the manufacturer to maintain the system's integrity.
If the air entry is caused by leaks in brake components, such as lines, hoses, calipers, wheel cylinders, or the master cylinder seals, I replace the damaged or worn parts.
After component replacement, it's crucial to properly bleed the system to remove any trapped air.
To prevent air from re-entering the brake system, I always emphasize the importance of regular maintenance to my customers:
Use the correct brake fluid type specified by the manufacturer.
Avoid opening the brake fluid reservoir unnecessarily to prevent moisture and air from entering.
Flush and replace the brake fluid at the recommended intervals, typically every 2-3 years.
While repairing air in the brake lines is essential, preventing its recurrence is equally important. Here are some tips I share with my customers:
Regular inspections and maintenance are crucial to identify and address any potential issues with the brake system before they escalate.
During routine service intervals, thoroughly inspect the brake components for any signs of:
Leaks
Wear
Contamination
Replace the brake fluid at the manufacturer's recommended intervals or every 2-3 years, whichever comes first.
Avoid excessive braking that can cause the brake fluid to overheat and boil, potentially introducing air into the system.
Address any brake issues promptly, such as:
Unusual noises
Changes in pedal feel
Addressing issues early can prevent further damage or air entry.
The cost of repairing air in the brake lines can vary depending on the underlying cause and the extent of the repairs needed.
| Repair | Typical Cost Range |
|---|---|
| Simple brake bleeding procedure | $50 - $150 for parts and labor |
| Replacing components (brake lines, calipers, master cylinder) | $200 - $800 or more |
The cost can increase significantly if components like brake lines, calipers, or the master cylinder need to be replaced, potentially ranging from $200 to $800 or more, depending on the make and model of the vehicle and the specific parts required.
Air in the brake fluid system is a common issue that can have severe consequences if left unaddressed. As a mechanic, I've witnessed firsthand the importance of promptly identifying and resolving this problem to ensure the safety of my customers and their vehicles.
By understanding the causes, inspection methods, repair procedures, and preventive measures, vehicle owners can take proactive steps to maintain the integrity of their brake systems and avoid costly repairs down the line.
Remember, brakes are a critical safety component, and any compromise in their performance can put you and others at risk. If you suspect air in your brake lines, don't hesitate to seek professional assistance. Regular maintenance and addressing issues promptly can go a long way in preventing air entry and ensuring your brakes are always in top condition.
Air can enter the brake system through leaks in brake lines, hoses or components. It can also get in when replacing parts or topping up brake fluid if proper bleeding procedures are not followed.
Air compresses easily unlike brake fluid, so it reduces hydraulic pressure transfer to the brakes. This leads to a spongy pedal feel and longer stopping distances.
The main symptoms are a soft or spongy brake pedal that sinks towards the floor when pressed, and inconsistent or poor braking performance.
Press the brake pedal firmly and hold it down. If the pedal gradually sinks or feels spongy, it likely indicates air trapped in the lines.
The proper method is to bleed the brakes following the manufacturer's recommended sequence to purge any trapped air from the lines.
No, driving with air in the brake lines is extremely dangerous as it compromises your ability to stop the vehicle effectively.
Most manufacturers recommend changing brake fluid every 2-3 years or 24,000-36,000 miles to prevent moisture contamination.
Contaminated brake fluid can absorb moisture and air, lowering its boiling point and allowing air bubbles to form in the lines.
Yes, if the brake fluid level gets very low in the reservoir, air can get drawn into the master cylinder and brake lines.
Regularly inspect for leaks, maintain proper brake fluid levels, change fluid periodically, and properly bleed brakes after repairs or replacements.
Sarah isn't your average gearhead. With a double major in Mechanical Engineering and Automotive Technology, she dived straight into the world of car repair. After 15 years of turning wrenches at dealerships and independent shops, Sarah joined MICDOT to share her expertise and passion for making cars run like new. Her in-depth knowledge and knack for explaining complex issues in simple terms make her a valuable asset to our team.