Just as with any hydraulic system, power steering can develop pockets of trapped air in the lines and may need to be bled after part replacement, addition of a ram, a complete fluid flush, or even a rollover. Symptoms of trapped air in the steering system can be a noisy, groaning pump or increased difficulty in turning the wheel. Running a power steering system with air in it is a good way to ruin a pump in a short amount of time. While some vehicles may be different, here is a quick overview of what is required to bleed a power steering system.
The first step in bleeding a power steering system is to check for any damaged or leaking lines or seals that could possibly introduce air into the system. If all that looks sound, be sure to check and top off the power steering fluid to the correct level. If power steering fluid appears foamy, then you likely have air in the system.
Next, you will want to raise the front wheels off the ground and cycle the steering wheel from lock-to-lock several times with the engine off. Leave the reservoir cap on because fluid will likely burp into the engine compartment. Listen for the sound of air being purged as you cycle the steering. This may take up to 20 cycles or more. Between cycles, re-inspect the fluid reservoir and top off as needed. Once the fluid level stabilizes and no aeration appears in the reservoir, start your engine and see if your symptoms have been resolved.
In rare instances, your steering box may be equipped with a bleeder valve. Just as with a braking system, you will want to find a length of hose and a small bottle filled with fluid. Place one end of the hose over the bleeder and the other end in the bottle underneath the surface of the fluid.
With the bottle in place and engine running, crack the bleeder slightly and have a helper turn the wheel from lock-to-lock several times. Close the bleed valve and top off the fluid and repeat until air no longer enters the bottle. If you are working alone and you know the fluid is fresh, you can always run the tubing back to the power steering reservoir to avoid running the system dry.
Finally, recheck the fluid level and add any fluid if necessary. After sitting for a several minutes, start the engine and verify that no more bubbles appear in the reservoir. If the pump runs smoothly and quietly without any leaks or change in effort in the steering range, you are all done.