Brake fluid reservoirs and caps

Brake fluid reservoirs and caps are essential components of the braking system, ensuring the storage and proper flow of brake fluid throughout the system. The reservoir ensures proper system operation, while the cap protects the fluid from leaks and contamination. At Global Parts, you'll find a wide selection of used car parts, including brake fluid reservoirs and caps, perfectly suited for BMW , Audi , Mercedes , Volkswagen , Mini , and other brands .

What are brake fluid reservoirs and how do they work in practice?

Brake fluid reservoirs are components mounted directly on the master cylinder, storing the hydraulic fluid used to transfer pressure from the brake pedal to the brake calipers. In most passenger cars, the reservoir capacity ranges from 200 to 500 ml, depending on the system design (e.g., mid-range cars like the BMW 3 Series or Audi A4 typically have reservoirs of approximately 300–400 ml). Brake fluid caps perform a sealing and compensating function – modern caps are equipped with a venting membrane that equalizes pressure with temperature changes while simultaneously limiting moisture absorption. This is important because DOT 4 brake fluid can absorb up to 2–3% of water over two years of use, lowering its boiling point (from approximately 230°C to below 170°C). In terms of compatibility and fit, the diameter of the nozzle, the spacing of the mountings and the type of fluid level sensor (mechanical or electronic) are crucial.

What are the functions of brake fluid reservoirs and caps ?

Brake fluid reservoirs and caps play key roles in a vehicle's braking system:

• Brake Fluid Storage – The reservoir ensures constant availability of the fluid necessary for the proper operation of the braking system.
• Leak Protection – The reservoir cap prevents brake fluid loss, which could lead to brake failure.
• Protection against contamination – The cap protects the fluid from moisture and contamination that could reduce its effectiveness.

Technical parameters and construction types

Brake fluid reservoirs come in several design variants:

• integrated with the brake pump,
• press-fit assembly with rubber seal,
• equipped with a fluid level sensor (float or resistive).

Typical technical parameters

Parameter	Typical value	Practical importance
Capacity	200–500 ml	Provides a fluid reserve as the pads wear out
Material	DOT 3/DOT 4 resistant material	Chemical and thermal resistance
Operating temperature	-40°C to +120°C	Stability in winter and summer conditions
Cork type	With compensation membrane	Limiting moisture absorption

It's worth paying attention to compatibility with the brake master cylinder and fluid type (DOT 3, DOT 4, DOT 5.1 – excluding DOT 5 silicone, which requires a separate specification). Proper fit affects tightness, durability, and operational safety.

Why is it worth taking care of brake fluid reservoirs and caps ?

Failure of these components can affect the braking system's effectiveness and increase the risk of failure. Typical symptoms include:

• Brake fluid leaks – May indicate cracks in the reservoir or a leaking cap.
• Reduced fluid level – May be due to leaks or improper cap seal.
• Brake Fluid Contamination – A leaky cap can allow moisture or contaminants to enter the reservoir. If you need a replacement, you'll find the right used car parts at Global Parts that will restore full efficiency to your braking system.

Brake fluid reservoir and pad wear – technical relationship

It's worth knowing that the fluid level in the reservoir naturally decreases as the brake pads wear. This is because the caliper pistons extend further, compensating for friction material wear. In practice, a 5-15 mm drop in fluid level may be an operational phenomenon, not a malfunction. Therefore, brake fluid reservoirs are designed with adequate reserve capacity to ensure stable system operation even with significant wear of the friction elements. This demonstrates the importance of proper design and tightness of the brake fluid cap.

How to choose the right brake fluid reservoirs and caps ?

When choosing brake fluid reservoirs and caps, there are several key aspects to consider:

• Vehicle-specific fit – Every braking system has its own specific technical requirements. At Global Parts , you'll find brake fluid reservoirs and caps perfectly suited to BMW , Audi , Mercedes , Volkswagen , Mini , and more .
• Technical condition – Used components should be free from cracks, mechanical damage and traces of corrosion.
• Reservoir Capacity – Make sure the reservoir you choose has the appropriate capacity to meet the requirements of your vehicle’s braking system.

Matching procedure – step by step

To ensure full compatibility and correct installation:

1. Check the engine version and year of the vehicle.
2. Verify the brake master cylinder type (piston diameter e.g. 22 mm or 25.4 mm).
3. Compare the shape and arrangement of the connection stubs.
4. Make sure the tank has the correct type of fluid level sensor. This approach minimizes the risk of mismatch and allows the system to maintain its factory operating parameters.

Why choose used brake fluid reservoirs and caps ?

Used car parts , such as used brake fluid reservoirs and caps , is an economical solution that allows you to save money while maintaining high quality. By choosing used braking system components, you gain:

• Lower costs – Used components are much cheaper than new ones, but offer equally high quality.
• Ecology – Recycling parts supports environmental protection and reduces waste.
• A wide range – In our store you will find used brake fluid reservoirs and caps for the most popular car brands.

Comparison – brake fluid reservoir vs. hydraulic clutch reservoir

In some models (e.g., Volkswagen, Audi, BMW with manual transmission), the brake fluid reservoir also supplies the hydraulic clutch system. This means that one reservoir serves two hydraulic systems.

Technical differences

Element	Brake reservoir	Clutch reservoir (if separate)
Function	Brake caliper service	Clutch slave cylinder service
Capacity	200–500 ml	100–250 ml
Fluid type	DOT 3 / DOT 4 / DOT 5.1	Usually the same

Knowing this relationship makes it easier to plan replacements and build a complete shopping cart (e.g. brake master cylinder, brake lines, level sensor).

How to take care of brake fluid reservoirs and caps ?

To ensure that brake fluid reservoirs and caps function reliably for a long time, it is worth following a few rules:

• Regular inspections – Check the condition of the reservoir and cap during periodic technical inspections of the braking system.
• Maintain proper fluid levels – Replenish fluid regularly using high-quality products.
• Keep the system clean – Avoid getting dirt into the reservoir when adding fluid. If you need a replacement, Global Parts has the right used car parts to ensure the durability and reliability of your braking system.

FAQ – Frequently asked questions about brake fluid reservoirs and caps

Question	Answer
What is a brake fluid reservoir?	This is a component of the braking system mounted on the master cylinder, which stores the hydraulic fluid responsible for transmitting braking force.
What is a brake fluid cap?	This is a sealing element of the tank, often equipped with a compensating membrane and a vent, which protects the fluid against moisture and leakage.
What is the capacity of the brake fluid reservoirs?	Most often from 200 to 500 ml in compact and mid-size passenger cars.
Does a drop in fluid level always indicate a leak?	No. It may be due to wear of the brake pads and the natural extension of the pistons in the calipers.
How to check tank compatibility?	Please take into account the car model, year, brake master cylinder type and fluid level sensor type.
Can a brake reservoir operate a clutch?	Yes, in many vehicles with a manual transmission, one tank supplies both hydraulic systems.

Summary: Brake Fluid Reservoirs and Caps at Global Parts Automotive Store

Brake fluid reservoirs and caps are essential components of the braking system, ensuring braking efficiency and safety. By maintaining their technical condition, you can avoid braking problems and improve driving safety. The Global Parts automotive store invites you to browse our wide selection of original used car parts . Whether you're looking for car parts for BMW , Audi , Volkswagen , Mercedes, Mini , and more, you'll find them at competitive prices. We strive to ensure every customer finds the right parts for their car, and if you have any questions or concerns about selecting the right components, we're happy to help! Please contact us – we're here to help! For more information and interesting facts, visit our automotive blog .