Do you bleed brakes with the engine running?

No. In most cases you should bleed brakes with the engine off to avoid interference from the brake booster and to maintain a firm, predictable pedal. Always check your vehicle’s manual for model-specific guidance, especially if you have an electric or non-vacuum braking system.

Why the engine status matters

Most passenger cars rely on a vacuum-assisted brake booster that draws power from the engine. Bleeding the brakes while the engine is running can alter the booster’s behavior and the feel of the pedal, making it harder to perform a clean bleed and potentially masking air in the lines. Cars with electric or non-vacuum braking systems may not depend on engine vacuum, in which case the engine’s status during bleeding is less critical. When in doubt, follow the vehicle’s service manual.

Standard bleeding procedure (engine off)

Below is a common two-person bleeding procedure used on many traditional hydraulic brake systems with a vacuum-assisted booster.

1. Prepare the vehicle and supplies: brake fluid of the correct specification, clear plastic tubing, a catch bottle, wrenches, and safety gear. Wear eye protection and gloves.


2. Check and top up the brake-fluid reservoir with the correct type of fluid. Do not let it run dry during the process.


3. Attach a clear hose to the bleeder valve at the farthest wheel from the master cylinder and place the other end in a catch bottle.


4. Start with the farthest wheel (usually passenger-side rear) and work your way to the nearest (driver-side front). Repeat for each caliper or wheel cylinder.


5. Have an assistant firmly press the brake pedal several times and keep it depressed.


6. With the pedal held down, open the bleeder valve. Fluid and air will flow out; once the flow becomes clear and free of air, close the valve while the pedal remains pressed.


7. Release the pedal slowly, then repeat until only clear fluid runs from the bleeder and no air bubbles are seen.


8. Proceed to the next wheel in the recommended order and repeat the process. Keep the reservoir topped up throughout.


9. After bleeding all four wheels, remove the hose, replace the bleed screws with their caps, and top off the reservoir to the proper level.

Conclusion: When finished, test the brake pedal for firmness with the engine off. If the pedal still feels spongy, there may still be air in the lines or a leak that needs attention.

Alternative bleeding methods

Two common approaches besides the standard two-person bleed are gravity bleeding and the use of vacuum or pressurized bleeding tools. These methods can be easier for solo technicians or when access is limited.

• Gravity bleed: Use the same farthest-to-nearest wheel order but let fluid flow by gravity from the master cylinder to the bleeder. This method requires longer time and careful monitoring to prevent fluid loss from the reservoir.


• Vacuum or pressure bleeders: A hand-held or powered tool creates a vacuum or pressure to move fluid and air through the system. These devices can speed the process and reduce pedal pumping, but follow the device’s instructions and keep an eye on the fluid level.

Conclusion: Both gravity and assisted-bleeding methods can be effective, but consistency and cleanliness remain key. Always work with fresh fluid and monitor for leaks and contamination.

Special cases and cautions

Some modern vehicles use electric brake boosters or non-vacuum hydraulic systems. For these models, engine status during bleeding may be less critical, but you should still follow the manufacturer’s instructions. If your vehicle uses an integrated or electronic system, consult the service manual or a qualified technician to avoid unintended consequences.

Electric brakes or non-vacuum booster systems

Vehicles with electric brake boosters or direct hydraulic assist do not rely on engine vacuum. In such cases, bleeding can often be performed with the engine running if the manual allows it; however, many technicians still prefer engine-off bleeding to ensure a stable procedure and pedal response. Always verify with the specific vehicle documentation and proceed accordingly.

Summary

Bleeding brakes is a critical maintenance task that aims to remove air from the hydraulic lines and restore a firm pedal. As a general rule, perform the bleed with the engine off to avoid interference from the brake booster and to maintain predictable pedal feel. If your vehicle uses an alternative braking system (electric or non-vacuum), follow the manufacturer’s guidance and consider professional assistance if you’re unsure. Regardless of method, keep the reservoir topped, use fresh fluid, and check for leaks after the job is complete.

Do you bleed brakes with an engine on or off?

You should bleed the brakes with the engine off because the running engine provides vacuum boost, which can interfere with the bleeding process and potentially introduce air into the system. With the engine off, you can properly pump the brake pedal to build pressure and evacuate air from the lines. 
How to bleed brakes with the engine off

• Park and prepare: Ensure the vehicle is parked on a level surface, turn the engine off, and engage the parking brake. 
• Pump the pedal: Have your partner pump the brake pedal several times until they feel resistance. 
• Hold the pressure: Instruct your partner to hold the brake pedal firmly to the floor. 
• Open the bleeder valve: Open the bleeder valve on the caliper or wheel cylinder until fluid stops flowing and then close it. 
• Repeat: Continue this process until no air bubbles are visible in the fluid being expelled. 

What is the correct way to bleed your brakes?

Bleeding Your Car's Brakes Takes Just 5 Steps:

1. Step 1: Change the brake fluid.
2. Step 2: Loosen the bleeder valve.
3. Step 3: Attach a clear plastic tube to the bleeder bolt.
4. Step 4: Have a partner press down on the brake pedal to drain the fluid.
5. Step 5: Repeat until new, clear, bubble-free fluid comes from the bleeder tube.

Do you pump brakes with a car on or off?

You should pump the brakes with the car off after performing work like changing brake pads, but you should pump them with the car on in most other situations to check for functionality, but be aware of the brake booster effect. 
When to pump brakes with the car off

• After a brake job: If you've pushed the caliper pistons back to fit new pads, pumping the brakes with the car off is necessary to pump the fluid and seat the pads against the rotors. You should do this before driving to ensure a firm pedal. 
• During brake bleeding: When bleeding the brake system to remove air, the car should be off. The pressure from the brake booster (which is active when the car is running) can affect the process and could be unsafe. 

This video demonstrates how to pump the brakes after a brake job to seat the pads: 52sExpertVillage Leaf GroupYouTube · Dec 17, 2020
When to pump brakes with the car on

• To check for issues: Pumping the brakes with the car on can help you check if the power brake booster is working correctly. If the pedal feels firm at first, and then sinks when you start the engine, it means the booster is working. 
• In stop-and-go traffic: Pumping the brakes when the car is on is a normal and necessary part of driving to slow down or stop. It is not bad to do it in this context. 

What to avoid

• Continuously pumping brakes with the car on: This can lead to brake overheating in a situation like heavy downhill driving. In such cases, using a lower gear is recommended to control speed instead of "riding" the brakes. 
• Pumping the brake pedal all the way to the floor: This could damage the master cylinder. 

What not to do when bleeding brakes?

Here are some of the usual errors to avoid when bleeding the brakes:

1. Using the Wrong Type of Brake Fluid.
2. Spilling Brake Fluid on Brake Pads.
3. Overfilling the Master Cylinder Reservoir.
4. Stripping the Bleeder.
5. Letting Moisture and Debris In.
6. Wrong Bleeding Sequence.
7. Overtightening the Bleeder Screw.