The power steering system on the 165 is activated by having a controlled amount of play in the steering. Too little and you will have no activation, too much and the efficiency will fall or it will become one-sided.
Have an assistant rock the wheel back and forth in the play while you examine the system closely with the wheels in the straight ahead position.
In the SA position there is a 'high spot' in the mechanism so play between the steering wheel and drop arm from the box should be negligible. If there is play present it may be possible to abjust it out using the adjuster on the RH side of the box. Next check the drag link joints on the bar that runs alongside the engine. If wear is present either the ends or complete bar will need to be replaced. Remove the front grill door and observe the movement. You will see a large scre with a nut on it. Controlled movement between this arm and the arm underneath is necessary to actuate the steering. The photo will show everything. There is an adjustable barrel connected between one of the arms and the valve block on the cylinder. Look particularly for play in the barrel pins and ends. Play here is a problem. The amount that the spool is pushed and pulled should be approximately 1/16" to 1/8". The movement should be positive and equal. Moving on, look at the main vertical shaft. If it appears to be 'kicking' its probably due to wear in the bottom bush as well as the top one. New bushes should be fitted but should be reamed in line using the special MF reamer. You will see where the cylinder rod is anchored in the casting. Try to lift the rod up and down. You may see it doing this as your assistant operates the wheel. Check the ball joints on the steering arms and look for any gap under the steering arms at the top of the king pins. This will indicate wear in the thrust bearings at the bottom. Probably the biggest problem will be access to the reamer or getting your local MF agent to do just that for you.
If there is wear at the end of the rod, a shim can be made to take this out....it is essential.
Most other repairs can be done by you if you have a little experience. We are here to talk you through any problems. If the rest of the tractor is sound then the steering is certainly worth doing.
DavidP, South Wales