We just installed a new power steering valve, and the power steering worked well a couple of days until it suddenly drops out, quit random, and at different rpm. The steering gets really hard and after 10 minutes it's ok again. Also it has vibrated a couple of times.
you have to bleed the lines. the easy way to do this is to disconnect the return line, start the engine and have someone turn the steering wheel from lock to lock a couple of times. have someone else make sure the fluid level doesnt drop too low and have someone else catch the used fluid in some kind of container to prevent spillage, a 3 liter soda bottle does nicely. once the air is gone, you should have no issues after that.