The most common problem in power assist steering system is leaks.  Most of leaks are from high pressure selas, which hold the power steering fluid in the steering racks / boxes, and it is caused by aged deterioration.   When the fluid is pressurised, it is heated up, and it makes rubber seals stiff like plastic.  Therefore, the rubber seals can not hold the fluid anymore, causing leaks.

Also many O-rings are used in racks, boxes, and pumps, and they deteriorates as same as seals.  So it can cause leaks as well.

Besides, the scratches on the shaft of the steering rack cause leaks.  Dirty power steering fluid contains a lot of small particles, or dirt stucking on the bushes make scratches on the shaft.  When it is deep enough, it provides a gallary to fluid.

Split rubber boots can cause corrosion on the steering components, then leaks occur.  It is recommended to replace the boots, when it is found, not to damage seals. 

When you can hear noise from power steering components, it is most likely from the pump.  When the pump sucks air in the system, it starts making noise.  The possible cause of the noise is the hose between the reservoir and the pump, leaking main seal, faulty bearing, the seal or O-rings in the pump.

Most common reason is because of a weaken pump, as O-rings inside are squashed and become stiff, not flexible any more, then the pressure provided by the pump becomes not high enough to give a proper assist.

Also the faulty piston ring inside the steering rack can cause this problem.

Sometimes, it is because of the linkage of the steering system.

A play in the shaft occurs when the  rack pad behind the pinion shaft, or the bush on the left side of  the steering rack are worn.

When the right side of the shaft moves up and down, most likely the rack pad is worn, as some steering racks have a plastic rack pad. 

When the bush has wear, the left side of the shaft has a play.

Unfortunately, most of manufacturers do not provide these plastic parts separetely, however, we can make a new one with a special plastic.