Repair Information

In the safety system of a vehicle, the brake system is one of the most critical components, and brake fluid is the “blood” that keeps it functioning properly. As more drivers and repair shops recognize the importance of braking safety, brake fluid replacement has become a maintenance item that requires more attention.

The important point is that brake systems on current vehicles are not all the same. Traditional hydraulic systems, vacuum-assisted hydraulic systems, electronically controlled systems, EPB systems, brake-by-wire platforms, regenerative braking systems and pneumatic systems all have different structures. Because of that, the correct brake fluid replacement method also changes by system type.

This guide explains what brake fluid does, why it must be replaced, which brake system types are common, and how replacement methods differ between traditional hydraulic systems and modern electronic brake systems.

Foundation

Why brake fluid matters

The Role of Brake Fluid and the Necessity of Replacing It

Brake fluid plays a key role in transmitting pressure within the brake system. When the driver presses the brake pedal, the brake fluid quickly transfers force from the pedal to the brake calipers or brake drums, causing the brake pads to contact the brake discs or drums. This friction slows down or stops the vehicle.

Brake fluid must maintain stable performance under heat. Braking generates a significant amount of temperature, and the fluid must avoid boiling or vaporizing. If vapor forms in the brake lines, brake pressure can become insufficient and braking performance can drop. Brake fluid also helps protect metal components from corrosion, extending the service life of the brake system.

Replace Brake Fluid to Maintain Optimal Brake Function

  • Moisture absorption: Brake fluid absorbs moisture from the air. As moisture content increases, it can vaporize under high temperature and form bubbles in the brake lines. These bubbles make pressure transmission uneven, causing a soft brake pedal, longer braking distance and serious safety risk. A brake fluid tester is typically used to measure the moisture content in brake fluid.
  • Oxidation and degradation: Over time, oxidation changes brake fluid chemistry. Oxidized fluid may produce sediment or sludge that can block brake lines or brake system components, reducing flow and potentially leading to brake failure.
  • Contamination: The brake system is not completely sealed. Dust, impurities and other contaminants can enter the fluid during use, reducing performance and affecting normal brake operation.

Experts commonly recommend replacing brake fluid every 2 to 3 years or after driving 40,000 to 60,000 kilometers. This is only a general guideline. The actual interval should be determined by vehicle usage conditions and brake fluid test results.

If the vehicle is often driven in dusty, humid or high-temperature environments, or if it is driven aggressively, the brake system is under extra strain and the replacement interval should be shortened. If symptoms such as a soft brake pedal, longer braking distance or abnormal braking noise appear, check the brake fluid condition immediately and replace it early when needed.

System Types

Choose the method by brake architecture

Mainstream Brake System Types

01

Traditional Hydraulic Brake System

A widely used system in cars and motorcycles. It uses hydraulic force to amplify and transmit pedal force to the wheel brakes. Its reliability, cost-effectiveness and maturity make it a mainstream solution for passenger and commercial vehicles.

02

Vacuum-Assisted Hydraulic Brake System

Common on gasoline vehicles and some hybrids. A brake booster amplifies driver input, then the hydraulic system applies braking force. It remains dominant on fuel-powered vehicles, though new energy vehicles increasingly use electronic braking technologies.

03

Electronically Controlled Brake System

Systems such as ECB, ABS and ESP use electronic control to make braking smarter and more precise. The original article referenced rapid adoption and expected expansion as intelligent mobility develops.

04

Electronic Parking Brake

EPB upgrades the traditional handbrake with electronic control and intelligent functions. Its evolution reflects the trend toward electrified and integrated chassis systems.

05

Brake-by-Wire

Brake-by-wire replaces traditional hydraulic or mechanical transmission with electrical signals, offering faster response, lighter weight and greater intelligence for next-generation vehicles.

06

Regenerative Braking

Used in EV and HEV models. During deceleration or braking, the motor converts kinetic energy into electrical energy and stores it in the battery, improving range and reducing mechanical brake wear.

07

Pneumatic Brake System

Designed for medium and heavy commercial vehicles such as trucks, buses and construction machinery. It uses compressed air to transmit braking force and may include EBS and AEBS functions in modern vehicles.

Method Overview

Oil replacement methods by system

Brake Fluid Replacement Methods for Different Brake Systems

Brake fluid is the pressure and energy transmission medium in the brake system. As brake systems evolve from mechanical hydraulic systems to complex electronic control systems, brake fluid replacement has also changed. The correct method depends on control structure, module involvement, oil circuit design and whether the system requires a diagnostic tool.

This article mainly introduces two common replacement methods: a brake fluid bleeder method for traditional hydraulic systems and a diagnostic-tool-assisted method for electronic control systems.

Method 01

Hydraulic and vacuum-assisted systems

Brake Fluid Replacement Using a Brake Fluid Bleeder

This method applies to traditional hydraulic brake systems and vacuum-assisted hydraulic brake systems. These systems are suitable for most older gasoline vehicles and the majority of modern gasoline vehicles.

The replacement method for both systems is largely the same and includes two common approaches: gravity fluid replacement and pressure fluid replacement. Gravity fluid replacement uses liquid level difference so fluid flows naturally through the drain port. It is simple, but it may leave old fluid behind. Pressure fluid replacement uses specialized equipment to push or exchange fluid more efficiently and with less residue. The AUTOOL AST series provides the pressure fluid replacement method.

Main Steps for Fluid Replacement

  1. Bleed the device first. Make sure the oil reservoir on the vehicle is filled to the maximum level.
  2. Find a suitable adapter and connect it to the vehicle.
  3. Press and hold the start button for 5-6 seconds to build pressure. Once pressure reaches above 0.6 bar, release the button.
  4. Adjust the pressure, usually to 2 bar.
  5. After turning on the machine and adjusting the pressure, loosen the screw on the wheel cylinder or brake caliper.
  6. To judge whether old fluid has been drained, check the layering and color of the fluid in the waste oil bottle. The darker layer is the old fluid.
  7. After replacement is complete, tighten the screw on the wheel cylinder or brake caliper first, then turn off the machine. This sequence is important because incorrect shutdown can allow air into the system.

During replacement, check whether there is enough new fluid in the new oil bottle. The device may not stop immediately if the bottle runs out. If the bottle runs out but the device continues feeding fluid, air may enter the system. The low-oil shutdown function is meant to protect against completely draining fluid from the vehicle, but operators should still monitor the bottle throughout the process.

Method 02

Electronic control systems

Brake Fluid Replacement With a Diagnostic Tool and Brake Fluid Bleeder

This method applies to electronic control brake systems, electronic parking brake systems and regenerative braking systems. Because these systems may lack large brake boosters and traditional vacuum structures, technicians cannot always use only traditional bleeding methods. Equipment requirements are more demanding.

It is recommended to use the AUTOOL AST series brake fluid bleeder, which can steadily output replacement pressure of 2-3 bar. A diagnostic device, such as the LAUNCH X431 diagnostic tool, can be used together with the brake fluid bleeder. This combination supports ABS and EPB system fluid replacement and is suitable for many vehicle models.

Unlock the Electronic Parking Brake and Enter Maintenance Mode

  1. Position the vehicle in the workstation, lift it up, and press the brake pedal to manually release the EPB. Once released, the instrument panel indicator for the EPB turns off.
  2. Enter the vehicle infotainment system, go to Settings – Driver Assistance – Brake Service – Release Parking Brake, and select “Yes” without pressing the brake pedal.
  3. After these steps, a long “zzz” sound may come from the rear parking brake motor, and the dashboard may display a fault light for the EPB system. This indicates the system has fully disengaged. At this point, the engine can be turned off without further action.

Drain the Old Fluid and Replace With New Fluid

  1. Open the brake fluid reservoir cap and use a pneumatic oil extractor to remove old fluid from the reservoir. Be aware that the reservoir may have two layers, which can make complete draining more difficult. Once old fluid is drained, pour new brake fluid into the reservoir until full.
  2. Connect the brake fluid replacement machine to the container of new brake fluid, usually 2-2.5L of new brake fluid, and tighten the cap on the original vehicle reservoir.
  3. Replace the brake fluid using the traditional hydraulic brake method while the diagnostic tool manages the electronic system requirements.

Reset the System Using a Diagnostic Tool

After fluid replacement, some vehicles, especially electric vehicles or those equipped with EPB systems, require a diagnostic tool to reset the system.

  1. EPB system reset: enter the EPB function menu and perform “Install New Brake Pads” or “Exit Maintenance Mode.” The system automatically tightens the parking brake calipers. Once successful, the “Service Electronic Parking Brake” warning light turns off.
  2. Clear fault codes: return to the main interface, enter “Read Fault Codes,” and clear EPB or ABS error codes generated during replacement. Confirm no warning lights remain on the dashboard.
  3. Exit service state when required: some electric or hybrid vehicles must exit maintenance mode or high-voltage disconnection state before the vehicle can be started.
Final Check

Brake test before delivery

Final Check and Brake Test

After replacement is complete, perform the following checks to confirm the brake system functions properly.

Check ItemInstruction
Brake Fluid LevelOpen the fluid reservoir cap and ensure the level is between the MIN and MAX markings.
Pedal FeelPress the brake pedal multiple times. The pedal should gradually become firm and should not feel spongy or sink under pressure.
Caliper BoltsEnsure the caliper bolts are fully tightened and there are no fluid leaks around the brake system.
Drain Plug SealCheck all components for integrity to prevent oil or fluid leaks.
Dashboard Warning LightsMake sure no ABS, EPB or brake system warning lights remain illuminated on the dashboard.
Road Test Brake PerformanceConduct a short-distance test to check brake response and confirm normal braking performance.

Friendly Reminder

  • After replacing brake fluid, let the vehicle sit for 2-5 minutes before starting the engine.
  • Brake fluid is corrosive. If it contacts vehicle paint or components, rinse immediately with clean water and dry thoroughly.
  • Unused new brake fluid should be sealed and stored properly to prevent moisture absorption and degradation.
Troubleshooting

Common issues after replacement

Common Issues and Troubleshooting Table

Improper operation or mismatched tools can cause common issues during brake fluid replacement. Use the table below for quick troubleshooting, improved work efficiency and reduced rework.

Issue SymptomPossible CauseSolution
Soft pedal, weak brakingAir in the brake linesRe-bleed the system to ensure no air bubbles remain.
ABS light stays onABS pump not activated or residual fault codesActivate the ABS pump using a diagnostic tool and clear the DTC.
Old fluid color remains after replacementIncomplete fluid replacementUse the fluid exchange function for a longer period or activate the ESP module.
Unable to drain fluid from rear wheelsEPB calipers not releasedRelease the EPB system using a diagnostic tool.
Dashboard warning or brakes not working after replacementMaintenance mode not activated on electric vehiclesDisconnect high-voltage power or enter service mode before proceeding.
Conclusion

Conclusion

The brake system is central to vehicle safety, and brake fluid directly affects braking force transmission. As vehicle technology has evolved from ABS to EPB and brake-by-wire systems, brake fluid replacement is no longer only a simple process of draining old fluid and adding new fluid. It requires the right procedure, suitable tools, diagnostic support when needed and final verification.

For traditional hydraulic and vacuum-assisted systems, a brake fluid bleeder can efficiently complete pressure fluid replacement. For electronic control brake systems, EPB systems and regenerative braking systems, a diagnostic tool and brake fluid bleeder should be used together to enter service mode, release EPB functions, activate modules and clear fault codes.

Contact AUTOOL for expert brake fluid replacement knowledge and tool recommendations.

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