The power-steering pump is beltdriven off the engine and is what provides fluid pressure to assist the mechanical effort of the steering box. This is what allows you to turn the tires with a single finger on the steering wheel.
Such pumps are a rotary-vane design. That is, a set of sliding metal vanes spins on a rotor shaft, and the vanes move inside an elliptical chamber. This action pressurizes the fluid being pumped from the steering box.
The pump is designed to output a fixed fluid volume for each revolution of the pump. As such, the faster the engine speed, the greater the output volume. Pumps are designed to provide adequate pressure (approximately 1,000 psi) and volume for assisted steering at low engine speeds, and this means they provide excessive flow at high engine speeds. A relief valve is used to reduce the output pressure at higher engine speeds.
Here is a typical GM integrated-reservoir power-steering pump. Depending on the vehicle, the fluid reservoir may be attached to the pump directly or mounted remotely, above the pump level and often on the inner fender. | 
This pump mounts to an engine bracket using bolts that thread into the holes seen here on the front. This pump uses a steel pulley (removed) that is tightly press-fitted onto the main pump shaft. | 
You'll generally need a pump pulley kit to remove and replace the pulley. Replacement pumps are typically sold without a pulley, and often they are secured to the pump shaft with a press fit. Simple puller/pusher kits are relatively inexpensive. |
Repair
Whenever a power-steering system fails to operate properly, check the easy things first.
Check the fluid level in the reservoir and check the drivebelt tension. Fluid leaks at the pump may signify loss of pump pressure, resulting in poor steering boost. Obviously, a leaky high-pressure hose can cause the same problems.
Should you find the pump leaky or otherwise at fault it can be replaced, or it is possible in many cases to rebuild it if kits are available. These usually consist of seals and gaskets, and may also include a shaft bearing. For best results, take care in cleaning all parts and assemble in a clean work space.
Pulleys may be V-belt-style (shown here) or serpentine-belt-style | 
On the back of this pump assembly are two nutted studs that hold the pump in the reservoir. You can also see the low-pressure fluid-return tube that simply empties into the reservoir housing and the high-pressure outlet fitting that threads into the back of the pump. | 
Removing the outlet fitting reveals the pressure-relief valve and its spring which is used to bypass some of the fluid if pressure build is excessive. A sticky relief valve can cause loss of pressure at low engine speeds, resulting in steering firmness. |
System Bleeding
Anytime a steering box, pump, or hose is removed from the system, air will enter it. You'll need to make sure to bleed these pockets of air from the fluid path after the component replacement. We have seen several recommended methods for bleeding air from the power-steering system, so we suggest you follow the directions included with the replacement part.
Some manufacturers suggest you fill the reservoir with fluid and then run the engine for short periods of time and, working the steering wheel back and forth, refilling in between runs. Others suggest you jack the front tires off the ground and manually steer the tires back and forth without the engine running to work fluid into and air out of the system to prevent pump damage on engine start-up.
Check your vehicle manual or the instructions that come with your new pump. Many pump manufacturers specify that you use only certain types of power-steering fluid and not use auto-transmission fluid, as it may not contain needed additives and friction inhibitors.
With the rear nutted studs removed, the pump can be separated from the reservoir housing. A large O-ring seals these two components together. When dealing with sheetmetal reservoirs such as this, be careful not to distort the housing during disassembly. Doing so can result in leaks between the pump body and reservoir. | 
Here you can see the back side of the pump. A large metal cap held in place with a snap ring seals the internal pump components in the pump body. | 
The pump shaft has been pulled out of the back of the pump housing. It rides on a bearing on the front side of the casting. The assembly seen here on the pump shaft is the top and bottom plates and cam ring. |
Aftermarket Pumps
If you're running large tires or need more power assist, there are aftermarket pumps available that provide greater fluid flow and pressure. These can also be combined with specially modified steering boxes for even greater help turning big meats.
From the factory, many vehicles have a small steel-tube loop in the fluid path that serves as a mini-cooler. If you're working your steering aggressively, you may want to consider a supplemental inline cooler to increase fluid capacity and cooling.
Internally, we can see O-rings that separate the pump chambers and feed and outlet ports. Fluid bypassed by the relief valve is routed internally back to the fluid input. | 
This somewhat elliptical bore has two pumping pockets that lie 180 degrees apart. This offers a balanced assembly as the fluid is pumped. | 
The rotor has thin metal vanes that slide outward with centrifugal force to conform to the inner wall of the cam ring. The rotating vanes push and pressurize fluid inside the cam ring. |
This is a remote-reservoir pump whose fluid supply comes from a fender-mounted fluid can. It's amazing that such small pumps can provide such power for our steering systems. Mechanical engine energy is converted to fluid pressure used to mechanically push the power piston in the steering box. | 
Here's another view of the remote-reservoir pump. You can see the internal pump chamber with sliding vanes. The top plate in the housing in the foreground seals one end of the cam ring and rotor vane. | 
Fluid enters the lower-pressure side of the pump through the barbed tube fitting on the left. The low-pressure hose can be replaced with cut-to-length hose approved for use with power-steering fluid and held in place with screw clamps. A specially formed, threaded high-pressure hose is required at the pump outlet to contain the high-pressure fluid flow there. This pump mates to a splined pulley held in place with a washer and nut. |
With the power-steering pump often tucked somewhere down on the front of the engine, it may be a neglected component. At a minimum, check the fluid level periodically and consider flushing the system when the fluid becomes dark and dirty. | | |