Power Steering Cylinders & Control Valves

On modern cars, trucks, and SUVs there are two types of hydraulic power steering systems, both employing an engine driven power steering pump. On one system the pump supplies fluid to a rack & pinion steering gear, on the other fluid is pumped to a conventional recirculating ball steering box. However, in the 1950s through the 1970s there was another power steering system used on some vehicles in conjunction with a manual steering box, which was variously known as external control valve power steering, hydraulic cylinder power steering, linkage assist power steering, or power assisted steering. In this system, the control valve and power piston that are integral components of a modern power steering box, are separate, externally mounted parts.

In the external power assist system, the control valve is attached to the end of the center link and the pitman arm from the steering box is connected to a ball stud on the control valve. The power piston is encased in a hydraulic slave cylinder. A ball stud at one end of the power steering cylinder is secured at a point along the length of the center link. The hydraulic cylinder piston rod is attached to a bracket on the vehicle frame. The pressure and return hoses from the power steering pump are connected to the control valve, and two smaller pressure and return hoses connect the control valve to the hydraulic slave cylinder.

When the vehicle is driven straight ahead and the steering wheel is centered, the control valve applies equal pressure to both sides of the piston in the hydraulic cylinder. When the steering wheel is turned, the control valve directs fluid pressure to one side of the piston while the fluid on the other side flows back through the return lines to the control valve and pump reservoir. When pressure is applied to the piston, the piston rod extends or retracts, depending on which way the steering wheel is turned, enabling the hydraulic cylinder to apply force to the center link, thereby providing power assist for the driver.

Because it uses the existing manual steering box, external power steering systems could be adapted to many vehicles and some of the first systems were used to provide easier steering on trucks. External power steering systems were used on many car lines and it’s usually easy to gather the parts needed to upgrade a model with manual steering to factory power steering. These advantages notwithstanding, external power steering units have their downsides. These systems were not known for providing optimal road feel, and the external control valve and hydraulic cylinder create additional points in the steering linkage for potential play that can cause loose steering. But the biggest weakness in these systems are their mounting location on the underside of the vehicle where components are exposed to road salt and the elements that can cause rust and corrosion, and debris and obstacles that can cause physical damage.

Like any steering system, fluid leaks and loose steering feel are the most common external power steering system problems. On many systems the hydraulic cylinder piston rod is exposed and if it becomes marred it can in turn destroy the cylinder seal and create a leak. On some systems the rod is enclosed in a boot, but the boot can become cracked or torn and allow dirt and debris to contact the shaft. Check for leaks due to cracked, deteriorated or otherwise damaged hydraulic hoses and check for leaks at the control valve. If the steering wheel can be turned back and forth quite a bit without the front wheels moving, worn power assist steering components may be indicated. However, the steering column and steering linkage should be checked for wear and play before those parts are condemned. Check for play in the steering column shaft bearings, mounts, and coupling, and check the idler arm bushings and all steering linkage ball studs.

If all other components check out OK, and the excessive looseness is isolated to the power assist steering components, check the control valve ball stud for play that can be caused by worn bushings. Physical damage and corrosion can also result in binding inside the control valve. Check the power steering cylinder ball stud for looseness in the cylinder and where it mounts on the center link and check the piston rod bushings at the frame mount for looseness and deterioration. Control valves can be adjusted if the amount of pressure applied to the hydraulic cylinder piston is not the same for both right and left turns, which can cause pulling.

No matter what your repair needs are or what you’re working on, we have the replacement power assist steering components to restore safe, predictable, and easy steering system operation without leaks. We offer power steering cylinders and control valves that are manufactured to original equipment specifications. We also have control valve rebuild kits, power steering cylinder rebuild kits, control valve ball stud kits, control valve seal kits, power steering cylinder piston rod seal kits, boot kits, and power steering cylinder mounting kits.