Power steering works by way of hydraulic pressure. Fire up some hydraulic pressure in something like a backhoe, and moving giant rocks around becomes as easy as pulling a few levers. A forklift uses hydraulic pressure to lift pallets full of concrete blocks high onto this shelf or that. This miracle of hydraulic pressure also makes a great deal of the everyday driving experience easier. Turning the steering wheel from left to right in a car or truck is effortless thanks to power steering by way of hydraulic pressure. Things that work by way of hydraulics have no room for pneumatics. Power steering systems are one of these things. Air can be compressed whereas fluid cannot. Air has no place in a hydraulic power steering system.

Checking for air in the system
If the usual effortless power steering experience has become noisier and more laborious, then there could be some air in there. A sure sign of air in the system is what sounds like a mildly disgruntled cat under the hood. This growling will get louder during power steering-intensive movements such as parallel parking. The first thing to check when the power steering starts moaning and groaning is the fluid level. If topping off the fluid calms down the noise and returns power steering operation to normal, then all is well. If the groaning returns a short time later along with fluid gone missing – then suspect a leak as both the reason for the fluid vanishing act and air entering the system.

Bleeding the power steering system
The usual suspect in power steering fluid leaks is the power steering pressure hose. In the process of fixing where the air gets in, it’s easy to introduce some more. After replacing a power steering pump or pressure hose, it’s always a good plan to flush and bleed the power steering system of air. For most power steering-equipped vehicles, this is a simple process. Turning the steering wheel lock to lock several times will remove any unwanted air in there. In some machines, like the Mitsubishi Starion, the system requires bleeding the power steering beyond the usual back and forth routine.

How-to tips
The following are a few car maintenance tips on bleeding power steering systems from the service manual and the miracle of the Internet.

Step 1: First things first. Check the power steering fluid level. Check the fluid hot or cold, depending on what the manual says. Turn the steering wheel a few times and take a reading. Foamy fluid is an indication of air getting into the system.

Step 2: Add power steering fluid if the level is low. Check the owner’s or service manual for the type of fluid before adding anything.

Step 3: Find the bleed valve and apply some penetrating oil. Push a length of clear vinyl tubing on the end of the bleed valve. Tubing length is better too long than too short. Raising the wheels above the ground with a jack and jack stands can make turning the steering wheel easier.

Step 4: Run the tubing into a dedicated catch container to bleed the system of both air and old fluid. Be careful not to run the system out of fluid! Start the engine. Crack open the bleed valve slightly. Turn the steering wheel several times lock to lock. Close the bleed valve. Add fluid. Repeat until the fluid runs air free.

Step 5: If the fluid is known to be new and free from crud, then running the return line back into the fluid reservoir is an option that does away with the danger of running the system dry. Fasten the end of the tubing to the reservoir with a rubber band or bailing wire. Start the engine. Crack open the bleed valve slightly. Turn the steering wheel several times lock to lock. Top off the power steering fluid once the air is no longer in there. Turn the steering wheel a few more times and check the fluid level once more before driving.