How It Works - Power Cylinder

The Power Cylinder is a construction of one tube inside another. One end of the Cylinder generally has a Stud mounted into it where the Cylinder connects to the Centerlink. This Stud can swivel on the Cylinder to allow for linkage movement and to prevent binding. Some Cylinders do not have a Stud but an Eyelet for bushings where it mounts to the frame of the car. The other end of the Cylinder has a polished metal Rod coming out of it that attaches to a bracket mounted on the drivers side frame rail. The end of this Rod goes inside the inner tube of the Cylinder and has a Piston attached to its end. Where the Rod comes out of the Cylinder are a series of seals to contain the pressurized fluid inside and to seal out water and dirt from the outside. The Rod also has a corrugated, stretchable Acchordion Boot to seal and prevent damage from moisture, rocks and dirt.

Fluid flows from the Control Valve to the two ports on the Cylinder. Depending on the flow from the Control Valve, one port is pressurized and the other acts as a return port. The pressurized fluid pushes against the Piston which moves the Rod in or out of the end of the Cylinder, depending on which side of the Piston is pressurized by the valve. The Rod, which is mounted to the cars frame rail by a cast iron Frame Bracket, either pushes or pulls against the bracket. Since the Frame Bracket is securely mounted and does not move, the Cylinder itself moves toward or away from the bracket, moving the Centerlink with it. Moving the Centerlink moves the steering linkage which turns the wheels and steers the car.
Basically, the Cylinder, by extending or retracting the Rod, makes itself longer or shorter and pulls or pushes the steering linkage with it as it moves.

Excessive Wear
The Stud can become too loose inside the end of the Cylinder. It is not unusual for it to flop around when it becomes marginal, but if it can actually be pulled in and out a bit, then it is too loose. Driving with it in this condition will accelerate wear and cause a clunking noise when changing direction with the car.
The Piston inside the Cylinder can wear to the point that it no longer seals well the the inner cylinder wall. Pressurized fluid will leak past the piston, lowering the pressure inside and increasing the volume of fluid needed to work the Cylinder. The Cylinder will become weak and will not provide full power and assist to the system.
The seals where the Rod comes out of the Cylinder can wear out and allow fluid to leak. The seals can be damaged by age and heat, making them hard and brittle. They can also wear due to contamination and abrasion by dirt located on the Rod as it moves in and out. A torn or missing Stretch Boot will cause very rapid wear of the seals and Rod surface.

Corrosion
Often a torn Stretch Boot will allow water inside and hold it against the metal Rod. This can cause the Rod to rust which leaves its surface rough and pitted. If this happens on an area of the Rod that goes through the seals on the end of the Cylinder, the seals will be quickly damaged and worn.
Water can get inside the power steering system and will often settle inside the Cylinder since that is the lowest place in the system. Water will rust the surface of the inner tube wall where the Piston rides, causing pitting that will quickly damage the Piston. You can often feel the roughness as the Piston slides over these pitted areas when you move the Rod in and out by hand.
Water will also get in the area where the Rod connects to the Frame Bracket. It will seep inside the Bushings and Metal Sleeve where it will rust up. Often the Metal Sleeve will become rusted to the end of the Rod and will be hard to remove and replace. Sometimes the corrosion in this area will be so severe that the metal Rod will begin to rot away inside the Bushings until it becomes so thin it may break during use.

Physical Damage
Improper removal of the Stud from the Centerlink (typically with a "pickle fork") will often loosen the Std inside its socket on the Cylinder. Removal by this method will almost always tear the Rubber Boot protecting the Stud. Removal by hammering on the end of the Stud will damage the threads and make it difficult to repair due to flaring of the end of the Stud.
While it is common to see dents on the tube of the Cylinder, this may not indicate internal damage. The Piston slides in a tube located inside of the outer tube, so unless the dent is quite deep, the inner tube may not be affected.
The Rod can easily be bent if the Cylinder is hard hit or bottomed out on something. The Rod comes out of the Cylinder through a small metal hole inside the Snout. The Rod is kept centered in the Snout by the seals, cup and washer located inside. This keeps the Rod from rubbing on the metal surface of the hole where it comes out. If the Rod becomes bent, even a little, the Rod will start to rub against the steel opening around the hole and will wear rapidly. The Rod has a hard-chrome surface and this will wear away or peel over time.
The end of the Rod where it mounts to the Frame Bracket can become bent. Often it will bend right where the threaded diameter area meets the larger diameter polished area. Sometimes this will cause the metal to crack and bending the end of the Rod back will cause the metal to tear even more, making it likely that it will break under use.