Steering gear box lube?
 

My '80 has power steering but there is a slight clunk at one point during low speed turning/cranking of the steering wheel. The ball joints, arms, etc., have been checked by a shop. All checks out fine.

There appears to be a lube nipple (like a zirk fitting) on top of the steering gear box. Is that a breather? No one has ever put lube in there as far as I can tell. Does it need some periodically?