How It Works - Control Valve Assembly

The Ball Stud is connected to the Steering Box by the Pitman Arm. The Ball Stud sets inside a Bushing that slides back and forth inside the Sleeve. The Ball Stud sets inside a pair of Ball Stud Seats which are held against the Ball Stud by pressure from the Bumper Spring. The Bumper Spring is compressed by the act of screwing the Travel Stop into the end of the Bushing.
The Spool Valve sets inside the Housing. When everything is assembled, the Spool Valve is kept centered in the Housing by the proper adjustment of the tension on the Centering Spring.

The Ball Stud can slide back and forth inside the Sleeve due to the movement of the Bushing. The Ball Stud is basically connected to the Spool Valve through the action of the Long Bolt, which changes tension on the Centering Spring. When the Ball Stud is centered in the Sleeve, the Spool Valve is centered in the Housing. With the Spool centered, the pressurized fluid from the Pump goes around the Spool Valve evenly and does not cause any steering action. When you turn the steering wheel, the Steering Box moves the Pitman Arm which moves the Ball Stud in the Sleeve off-center. When the Ball Stud moves off-center, it causes the Spool Valve to off-center in the Housing. This action lets the Spool Valve send pressure to the Power Cylinder which mounts between the Centerlink and the frame of the car. The Power Cylinder then pushes or pulls against the frame, forcing the Centerlink to move the steering linkage, thus turning the car.
As the Centerlink moves, it begins to move the Sleeve until the Ball Stud is once again centered in the Sleeve. Once this happens, the system is all centered again and no hydraulic action takes place. Basically, the power-assist system works by constantly trying to re-center itself whenever the steering wheel makes it go off-center. It is a consequence of this action that the steering linkage moves and turns the car.

Of course, there are many other actions going on in the Control Valve/Ball Stud assembly as well.
The Ball Stud sets between two concave metal seats that allow it to swivel around in all directions to a small degree. The Bushing slides back and forth inside the Sleeve, but also has some side-to-side movement as well. These areas of movement keep the Ball Stud from binding as the Centerlink moves in several different directions as the steering linkage and suspension move around.
There are a couple of seals on the Spool Valve and a couple of O'rings on the Reaction Valve Plugs that keep the pressurized fluid inside the Housing.
There is a restrictor inside the Housing to slow the pressure buildup in the system. In early models, it is a ball check valve inside the return hose port, on later models it is a restrictor plug inside the pressure hose port.
Most models have a Reaction Valve inside the Housing which is spring loaded on both sides.

There is basically only two adjustments on the Control Valve/Ball Stud assembly.
The Travel Stop is threaded into the end of the Bushing, compressing the Bumper Spring which puts tension on the seats that the Ball Stud sets in. This tension must be great enough to hold the parts together at all times, but there must be enough compression left in the Bumper Spring so that the Ball Stud is not binding. The Adjustment Nut compresses the Centering Spring so that the Spool Valve is centered inside the Housing under non-power situations. This adjusment must be correct or the Spool Valve will not center in the Housing and fluid will not go around it evenly. This will cause the power assist to favor turning one way over the other, or in extreme cases, actually want to turn the car and steering wheel by itself. The adjustment of this nut and the condition of the Centering Spring are important to prevent this from happening.

Excessive Wear
Because the Control Valve parts are constantly bathed in pressurized lubricant, they seldom wear out. However, the Ball Stud area is often neglected and is exposed to the elements more than the valve. The Bushing will wear first because it has a soft bronze outer layer, and it moves more and has more tension on it that any other part. The inside of the Sleeve where the Bushing slides will also wear. The inside of the Sleeve will develope ridges so bad that the Bushing will not slide properly inside. The metal tubing of the Sleeve will wear so thin around the Ball Stud opening that the metal will be razor sharp or crack. This problem is most common on early Galaxie models. The extra weight and strain of the big Fords put much more pressure on their Sleeves than those of the smaller cars this system is usually found on.

Corrosion
More common than damage from wear is that from rust and corrosion. This is true of the external parts exposed to water from underneath the car, and form water inside the hydraulic system itself.
Almost every valve will have some damage due to corrosion. Some parts will still function with some rust damage, but others must be replaced to operate properly. Corrosion on the Bumper Spring or Centering Spring will cause them to loose their tension which will affect the parts they provide tension on. Rust damage on the ball of the Ball Stud or its seats will cause metal flaking and looseness. Rust on the inside surface of the Sleeve will cause the Bushing to bind, wear excessively, and ultimately enlarge the inside of the Sleeve.
The most common corrosion damage occurs on the ends of the Spool Valve where it comes out of the seals. Any water that gets under the aluminum end cap or inside the Sleeve can find its way to the Spool Valve ends and cause them to pit, which tears up the seals and causes leakage. In Control Valves sent in for rebuild, about 50% have pitting on the Spool Valve.

Physical Damage
Some physical damage can occur to the parts of the Control Valve/Ball Stud assembly during its lifetime. Often there will be grooves worn into the surface of the Housing or the aluminum End Cap from the clamp bolts on the drivers side tie rod adjusting sleeve. If this adjustment sleeve is not rotated, and the bolts are not located in such a way as to prevent them from contacting the Control Valve during suspension and steering linkage travel, they can eventually wear through the Housing and into the hydraulics.
The Sleeve can be banged up in such a way that it is no longer round inside and the Bushing will no longer slide freely inside. On cars like the 1965-1966 V8 Mustang, which had the Sleeve area integral with the Centerlink, the Sleeve area can bend and will no longer be straight. This will also cause the Bushing to bind or lock up. Running over something that hits the Control Valve can push it up and bend the Sleeve area. The use of the wrong drivers side tie rod ends on the 1965-1966 V8 Mustang can cause the tie rods to slam up into the Housing under certain conditions, breaking the Control Valve off of the Centerlink.
Most physical damage is caused by the improper removal, installation and adjustment. Use of a "pickle fork" to remove the Ball Stud from the Pitman Arm will always distort the roundness of the Sleeve and cause it to be much more difficult to disassemble and repair. On models where the Control Valve/Ball Stud assembly threads onto the end of the Centerlink, installing it at the wrong position and/or location on the end of the Centerlink will cause improper operation. Removing a threaded-on assembly without first removing the Retaining Pin will cause the threads of both the Centerlink and Sleeve to be damaged. Over-tightening of the Adjustment Nut can cause the pin inside the Travel Stop to shear off.