Have replaced rack and pinion, power steering pump, reservoir (with clean screen) and pressure switch. Can I jumper 12v (and ground) to the switch to see if that is it, or would that be too much "juice"? Don't have a multimeter and can't seem to figure out what to try next

 

Either low on fluid, you may have the lines backwards on the rack and pinion, or the belt driving your pump is slipping.