The most common failure in a steering rack is the seals. Most steering racks use O-rings to seal the internal components. There are two separate systems at work inside a power-assist rack & pinion: the hydraulics (power assist), and the mechanics (steering). The two systems share the pinion gear inside the upper portion of the main housing. The pinion gear runs through the hydraulic servo. As the pressurized fluid runs through the valve, any movement of the pinion gear (which is connected to the steering column) rotates the spool valve to line up with the correct flow ports. When the steering wheel is centered (neutral position), the fluid bypasses the spool valve. The spool valve directs the pressurized fluid to the appropriate side of the assist piston inside the long tube of the steering rack. This provides the assist for steering. There are numerous lines and valves that need an O-ring seal. Because they are under severe pressures exceeding 2000 psi, they are prone to failing.
The mechanics of the rack are certainly more simple, but nonetheless capable of failing. The pinion gear rides on the rack (which is attached to the piston). There is a mesh setting, this can be get out of alignment through wear or physical damage, and cause hard or loose steering. Some racks have the steering shafts coming out of the end of the unit, over time these can get damaged or even strip out the threads, which requires complete replacement of the steering rack unit.
It is possible to rebuild the seals on a steering rack, but the process is lengthy and the deeper internal seals are more difficult to change without special tools. Replacement is typically the most cost effective solution. The process for replacing a rack typically takes 4-6 hours, make and model variances differ or course. Removing the rack involves disconnecting the steering linkage, removing the tie rods, disconnecting the hydraulic lines, and then the rack itself. There are two types of rack mounts- bushed lugs or clamps. This rack shown below (from a 2009 Dodge Challenger) uses the bushed lug style. Here is the rack itself. Note the two large lug bosses just to the inside of the bellows. These are the rack mounts. This is an end-steer rack, where the steering arms come out of the ends. The other style of rack is center steer, where the steering arms are mounted to a tie rod bar.