Should You Bleed a Power Steering System With the Cap On or Off?

In most cases, you bleed a power steering system with the reservoir cap off (or loosened) during the initial bleeding so trapped air can escape and you can monitor and top up fluid; cover the opening with a clean rag to prevent splashing. Once bubbles stop and the level stabilizes, install the cap and complete final checks. Always follow the vehicle’s service manual, as a few manufacturers specify installing the cap during the engine-running phase to reduce aeration.

Why the Cap Position Matters

The reservoir cap affects how easily air can vent, how well you can observe and maintain the fluid level, and how much mess you’ll make. Bleeding with the cap off helps air purge faster and lets you see when foaming subsides, but return flow can splash—especially once the engine is running. Many caps are vented, so leaving the cap on loosely can still allow air to escape while reducing spray.

A Practical, Generic Bleeding Procedure

The following steps outline a widely accepted method that balances safe venting with minimizing aeration. Always verify specifics for your make and model, especially fluid type and any OEM-mandated cap position.

1. Confirm system type: Ensure your vehicle uses hydraulic power steering (not electric EPS). If hydraulic, inspect hoses, pump, and rack for leaks or damage.
2. Raise the front wheels: Support the vehicle securely on jack stands so the front tires are off the ground, reducing steering effort and pump load.
3. Fill to the “COLD” mark: Use the manufacturer-specified fluid (PSF or ATF as called for). Do not overfill.
4. Engine OFF, cap OFF (or loosened): Cover the reservoir opening with a clean rag. Slowly turn the steering wheel from lock to lock 20–30 times. Do not hold it against the stops. Top up fluid as the level drops.
5. Rest period: Let the vehicle sit 5–10 minutes so microbubbles can rise out of the fluid.
6. Start the engine at idle: If your system tends to “fountain” at the return, install the cap or keep it loosely fitted; otherwise, keep the cap off with a rag over the opening. Slowly turn lock to lock 10–20 times, watching for bubbles/foam. Do not hold at the stops.
7. Manage foaming: If you see heavy foam or the pump whines, shut off the engine and repeat Step 4 (engine off cycling) until foaming subsides.
8. Stabilize the level: Maintain the fluid between MIN and MAX throughout. Once bubbles stop and noise is gone, install the cap fully.
9. Road test and recheck: Lower the vehicle, drive briefly, then recheck the level and look for leaks. Top up to the “HOT” or operating range mark if applicable.

This approach lets trapped air escape efficiently without making a mess, while protecting the pump from ingesting foam. The key is slow steering inputs, correct fluid, and patience.

When to Keep the Cap On During Bleeding

Some systems are prone to return-line “geysering” once the engine is running. In these cases, keeping the cap on (or lightly installed) during the engine-on phase reduces aeration and fluid spray. Always defer to the vehicle’s service manual; certain OEMs specify the cap installed after the initial, engine-off cycling.

Examples of Special Cases

The following circumstances may change the recommended cap position or bleeding steps:

• High-flow or remote reservoirs that splash under idle return flow.
• Hydroboost brake systems sharing power steering fluid, which may require additional cycling of the brake pedal.
• Vehicles with electro-hydraulic pumps that specify unique bleed routines.
• Fluid with anti-foaming additives where OEMs advise minimizing exposure to air during the engine-on phase.

If your system matches any of these scenarios, check the factory procedure before proceeding to avoid prolonged aeration or pump damage.

Common Mistakes and How to Avoid Them

These frequent errors can prolong bleeding time or harm the system. Use the checklist below to stay on track.

• Using the wrong fluid: Many systems require a specific PSF or a particular ATF; the wrong type can cause noise or seal damage.
• Turning the wheel too fast or holding at the stops: This can churn the fluid and pull more air into the system.
• Letting the reservoir run low: The pump can ingest air, causing foaming and cavitation.
• Skipping the rest period: Microbubbles need time to rise and escape.
• Bleeding with the front wheels on the ground: Increases load and aeration risk.
• Ignoring leaks: Air will re-enter if suction hoses or seals leak, making bleeding ineffective.

Avoiding these pitfalls speeds up the process, prevents pump damage, and yields a quiet, responsive steering feel.

How to Tell When the System Is Fully Bled

Indicators that you’re done include a clear, foam-free reservoir, a stable fluid level, normal steering effort with no shudder, and no pump whine. After a short drive, recheck the level and ensure there are no new bubbles or leaks.

Bottom Line

Bleed with the cap off (or loosened) during the initial, engine-off phase to let air out and to monitor level; cover with a clean rag to control splash. For the engine-running phase, follow the OEM guidance—if none is available, installing the cap (or keeping it loosely on) typically reduces aeration. Finish by securing the cap once bubbles stop and the level stabilizes.

Summary

Most power steering systems bleed best with the reservoir cap off or loosened at first, then on once the engine is running and the fluid is circulating. Use the correct fluid, cycle the wheel slowly with the front end raised, don’t let the level drop, and follow your vehicle’s service manual for any cap-on requirements during the running phase.

How do you get air out of power steering?

To remove air from a power steering system, fill the reservoir, lift the front wheels off the ground, start the engine, and slowly turn the steering wheel from full left to full right about 15-20 times while keeping the fluid topped off. Do not hold the wheel at the stops. Shut off the engine, recheck the fluid level, and repeat the process until the fluid level remains stable and the system is no longer noisy.
 
Tools and Materials

• New power steering fluid
• Jack and jack stands
• Wheel chocks
• Vacuum pump (optional, for stubborn cases)
• Liquid vapor separator (optional)

Step-by-Step Guide

1. Fill the reservoir: With the engine off, fill the power steering reservoir to the correct level. 
2. Lift the vehicle: Use a jack to lift the front of the vehicle, ensuring the front wheels are off the ground. Place jack stands for safety and then position wheel chocks at the rear wheels. 
3. Start the engine: Start the engine and let it idle. 
4. Turn the steering wheel: Slowly turn the steering wheel from the full left position to the full right position and back again, repeating the movement 15 to 20 times. 
5. Check and refill the fluid: As you turn the wheel, the fluid level will drop; add fluid as needed to prevent new air from entering the system. Do not let the level get too low. 
6. Listen for noise: You may hear gurgling sounds as the air is pushed out of the system. 
7. Repeat if necessary: If the system is still making noise or the fluid level continues to fluctuate, repeat the process. 
8. Final check: Once the fluid level is stable and the noises cease, shut off the engine and recheck the fluid level one last time. 

If Air Persists

• Vacuum bleeding: Opens in new tabFor systems with stubborn air, a vacuum pump can be attached to the reservoir to draw air out while the engine idles. 
• Check for leaks: Opens in new tabCheck for any external leaks, as an external leak can introduce air into the system. 

This video demonstrates the process of bleeding air from the power steering system: 57sCARDONE IndustriesYouTube · Nov 21, 2014
Important Considerations

• Safety: Always use jack stands to support the vehicle, never relying solely on a jack. 
• Fluid type: Use the correct type of power steering fluid specified in your vehicle’s owner’s manual to avoid damage. 
• Do not overfill: Overfilling the reservoir can cause fluid to spew out. 
• Avoid holding the wheel at stops: Turning the wheel to the full lock position for extended periods can put excessive strain on the power steering system. 

Do you leave the cap off when bleeding power steering?

You should bleed the power steering system with the reservoir cap off to allow trapped air to escape. The process involves filling the reservoir with the correct fluid, then slowly turning the steering wheel from full left to full right multiple times with the engine running, while continuously monitoring and topping off the fluid level to prevent new air from entering the system.
 
This video demonstrates the process of bleeding power steering fluid: 52s4DIYersYouTube · Jul 1, 2016
Steps to bleed the power steering system:

1. Prepare the vehicle: Park on a level surface, chalk the rear wheels, and lift the front end with a jack and jack stands so the front tires are off the ground. 
2. Locate the reservoir: Find the power steering fluid reservoir under the hood. 
3. Fill with fluid: Remove the reservoir cap and fill the reservoir to the manufacturer-recommended level using the correct power steering fluid for your vehicle. 
4. Start the engine: With the reservoir cap still off, start the engine. 
5. Turn the steering wheel: Slowly turn the steering wheel from full left to full right, making several complete cycles. 
6. Monitor fluid level: As you turn the wheel, check the fluid level and add more fluid as needed to keep it from dropping below the minimum level. This prevents more air from being sucked into the system. 
7. Repeat until clear: Continue this process until the fluid in the reservoir is no longer bubbling or foamy, which indicates the air has been purged. 
8. Top off and finish: Once the air is out, shut off the engine, top off the fluid to the correct level, and replace the reservoir cap. 

Important considerations:

• Use the right fluid: Always use the power steering fluid recommended by your vehicle’s manufacturer. 
• Don’t hit the stops hard: Avoid turning the wheel so forcefully that it hits the steering stops, as this can cause more air to enter the system. 
• Check for bubbles: If the fluid looks bubbly or foamy, shut the engine off and let it sit for a few minutes to allow the foam to settle before repeating the bleeding process. 

How to properly bleed a power steering pump?

To bleed the power steering pump, start with the engine off and reservoir filled with fluid. Loosen the return line or bleed screw to allow trapped air to escape. Slowly turn the steering wheel from lock to lock several times while monitoring fluid flow. Tighten the line once no air bubbles appear.

Do you bleed power steering with engine running?

Open your garage, start your car, and let it idle for about 20 minutes to bring the engine to operating temperature. Then turn the steering wheel to the left and right about 15 times. This will push air from the system. Shut off the engine, open the hood and check the steering fluid reservoir.