Common Brake Fluid Reservoir Cap Problems

Brake fluid reservoir cap: The brake master cylinder, also known as the master cylinder, is a hydraulic pump. Supplies brake fluid to the brake circuit to convert the pressure on the brake pedal into hydraulic pressure.


There are many moving parts that make up the brake master cylinder, and because it is the heart of the brake system, its health and longevity are important. We’ll explain the role each part plays, making it easier to identify a problem.

For every car that has disc brakes on the front wheels and drum brakes on the rear wheels, they will also contain a metering valve. The purpose of this valve is to indicate that the drum brakes come on first, followed by the disc brakes.

Next are the primary and secondary pistons. When you start to press the brake pedal, the main piston is activated and also begins to press down. Pressure will begin to build in the master cylinder as you apply more pressure to the brake pedal. This build up of pressure between the primary and secondary piston will cause the secondary piston to compress the fluid.

The filler cap attached to the brake fluid reservoir has a small hole at the top that allows the fluid to flow without causing pressure. It is important to keep this cap sealed to ensure that dust or moisture does not seep in and contaminate the fluid.

The fluid level sensor helps keep track of how much fluid is in the brake reservoir. When a pressure imbalance occurs, the brake light comes on, indicating that the reservoir needs to be checked.


Now that we know how the brake master cylinder works, it will be easier to troubleshoot. If your vehicle begins to experience any of these problems, have it checked by a transmission specialist found at local transmission shops.

Contaminated Brake Fluid

As we mentioned earlier, it’s important to keep the filler cap sealed to prevent contamination. When checking the brake fluid, if the color is a dark brown or black shade, it has been contaminated. New brake fluid should be clear or yellow.

Brake Pedal Not Working Properly

If the brake pedal feels spongy or begins to sink into the floor of the vehicle, it’s time to inspect the brake master cylinder.

Preventing Brake Master Cylinder Leaks

If the brake master cylinder or surrounding areas begin to wear and leak, the brake fluid level will begin to drop. These failures will cause the hydraulic pressure to drop and the pedal to touch the floor. 

There are two types of leaks that a master cylinder can experience: an external brake fluid leak or an internal leak from a damaged piston seal. Testing is a crucial step in preventing this. Fluid leaks of any kind can put you and others in danger, so be sure to consult an expert immediately.


Here is a step by step guide to properly test the operation of the brake master cylinder:

Step 1 – You will begin by opening the hood of your car and locating the brake fluid reservoir. If you are having difficulty finding the brake fluid reservoir, try looking towards the back of the engine compartment.

Step 2 – Have a friend or family member sit in your car and hit the brakes. While they are braking, watch the cylinder to see if the fluid begins to swirl or bubble. If so, you’ll want to bring your car in and have the brake master cylinder inspected.

Step 3 – Next, you’ll want to check the surrounding areas to make sure there are no leaks.

Step 4 – Start applying pressure to the brakes and continue to push down, maintaining the same amount of pressure until the pedal stops. If you notice the brake pedal beginning to sink deeper into the floor, you should call and make an appointment with any qualified transmission shop in your area. Most likely, your brake master cylinder will need to be repaired or replaced.

What is the brake fluid reservoir?

The brake fluid reservoir is a container attached to your brake master cylinder.

It stores your vehicle’s brake fluid and protects it from contamination by dirt and moisture, helping your car’s brakes perform at optimal levels.

Brake fluid reservoirs are usually made of polymeric plastic and can usually be separated from the master cylinder body. 

The reservoir cap is vented and has a diaphragm that expands and contracts to maintain normal pressure inside. There’s also a fluid level sensor that alerts you when brake fluid levels are low. 

3 Common Brake Fluid Reservoir Cap Problems:

Here are 3 common brake fluid reservoir problems:

  1. Plastic Reservoir Damage

The body of the plastic reservoir can crack and break over time. When this happens, the brake fluid will leak. 

Reduced brake fluid levels will, in turn, compromise your vehicle’s braking performance. 

These cracks will also allow moisture to enter and contaminate the brake fluid, accelerating brake fluid degradation.

  1. Malfunction

of the fluid level sensor If the fluid level sensor fails, you will not receive an alert when the brake fluid level falls below the minimum threshold. 

Not knowing when your brake fluid levels are low (and when you need to add brake fluid) will increase your chances of total brake system failure and compromise the safety of your vehicle.

3. Reservoir cap diaphragm worn When the reservoir cap diaphragm is in good condition, it keeps moisture out and allows the brake fluid level to drop naturally as the brake pads wear. 

However, a worn diaphragm can leak, allowing moisture and air to enter the brake fluid and ultimately the brake lines. 

While moisture contaminates brake fluid, air can also create air bubbles in the brake line, reducing hydraulic pressure and brake performance.

3 Symptoms of a Bad Brake Fluid Reservoir

These symptoms usually accompany a bad brake reservoir:

A brake fluid leak – You may notice a puddle of brake fluid under the front of your vehicle where the cylinder is located teacher.

Brake light comes on: A low brake fluid level will cause the brake light on the dash to come on.

Degraded Brake Fluid: As brake fluid absorbs water, moisture contamination will slowly darken the color of the fluid until it resembles used motor oil. 

The Easiest Way

to Repair the Brake Fluid Reservoir Replacing the brake fluid reservoir does not involve simply removing the reservoir from the master cylinder. You’ll need to get the correct fluid reservoir for your car, install it correctly, possibly bleed the car’s brakes, etc. 

Since there are so many steps involved, it’s best to find a qualified mechanic to do it for you.

And whenever you’re looking to repair your brake fluid reservoir, choose a mechanic who: 

Is an ASE Certified Brake Technician.

Use high-quality parts and tools.

Offers a service guarantee.

Fortunately, RepairSmith ticks all of those boxes.

What is RepairSmith?

RepairSmith is a convenient mobile vehicle maintenance and repair solution that offers you these benefits:

Brake replacements and repairs can be done right your way

Up-front, competitive pricing

Online booking is convenient and easy

ASE-certified expert technicians perform vehicle inspection and service

Repairs are performed using high-quality equipment, tools and parts

RepairSmith provides a 12-month, 12,000-mile warranty on all vehicle repairs

How much can you expect to spend?

The cost will depend on the repairs needed, your location, and the make and model of your vehicle. For a more accurate estimate of the costs involved, simply complete this online form.

Brake Fluid Reservoir Frequently Asked Questions

Here are some answers to some questions you may have about brake fluid reservoirs:

1. What does brake fluid do?

Brake fluid is the hydraulic fluid that acts as a conduit for hydraulic pressure in the brake line. 

When you step on the brake pedal, that force is amplified through a brake booster to the brake master cylinder. 

The brake master cylinder then pumps hydraulic fluid from the brake fluid reservoir into the brake line, converting pedal force into hydraulic pressure, activating the brake calipers. 

Each caliper then clamps the brake pads onto the brake rotors, bringing the wheels to a stop. 

2. What are the signs of low brake fluid levels?

Here are some signs that you are low on brake fluid:

The brake warning light comes on. 

Worn and squeaky brake pads. Worn brake pads cause the caliper piston to extend further to squeeze the rotor, drawing more brake fluid into the brake line.

A spongy brake pedal is a telltale sign of a low brake fluid level. 

Puddles of brake fluid under your car is another sign that you have a leak.  

3. How can I check the brake fluid level?

It is best to let a mechanic take care of this. 

However, if you really need to check the fluid level, here’s how to do it:

  1. Position the car. Make sure the vehicle is parked on a level surface, then open the engine hood.
  2. Locate the brake fluid reservoir. It is usually mounted near the firewall at the rear of the engine compartment, near the brake pedal.
  3. Check the deposit type. Newer vehicles may have a translucent brake reservoir so there is no need to open the lid. If the tank is cloudy, wipe the top to prevent dirt from entering before opening the lid to see inside.
  4. Check the fluid level. The brake fluid level should be between the MIN and MAX marks. 
  5. Check the color of the brake fluid. New brake fluid is clear and typically amber in color. Old brake fluid has a muddy brown or black color, like old motor oil, due to dirt and debris contaminating it.
  6. Replace the cover if you removed it. Do not leave the brake fluid reservoir open for too long as the brake fluid will begin to degrade.

Some IMPORTANT notes:

Brake fluid is toxic, so do not touch it.

Brake fluid is corrosive and can ruin your car’s paint.

If your vehicle has ABS, check your vehicle’s owner’s manual. Some ABS systems require pumping the brake pedal 25-30 times before checking the brake fluid level.

4. Can I add more brake fluid if the levels are low?

Yes, you can add brake fluid if you find that you are running low on fluid. 

However, make sure it’s from a new container and that it’s the correct DOT type of brake fluid, since you can’t mix different brake fluids. Also, remember that brake fluid is toxic and corrosive, so handle it with care.

If the brake fluid reservoir is empty, it will also be necessary to bleed the air from the brakes. And if the vehicle’s brake fluid is above the MAX mark, it may have water in it.  

In these circumstances, it is better to have a mechanic help you solve the problem.

5. How often should I change the brake fluid?

You should change your brake fluid every two years, or according to the brake fluid manufacturer’s recommendations. 

Sticking to a brake fluid change schedule protects your hydraulic system from internal corrosion and reduces the risk of premature brake system failure.

6. Can I change the brake fluid myself?

Although you can change it yourself, it is recommended that you let a professional mechanic do it for you. Changing brake fluid can require many different tools, some of which are specific to the process.

While you may be able to remove some of the old fluid from the brake fluid reservoir with a turkey baster, you’ll still need to properly replace the old brake fluid with new fluid and bleed the brakes to avoid air bubbles.

7. What does it mean to flush the brake system?

Flushing the brake system refers to the process of removing old brake fluid from the brake lines and replacing it with new fluid. This new brake fluid will improve the performance of your vehicle’s braking system.

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