dumper wrote: get the head off and have a look
Yes.
An autopsy will show.
It is necessary to check up a condition of directing plugs of valves of saddles of valves.
It may be necessary to replace the guide bushings.
Then you will need to prune the valve seat.
I would replace all valves, springs, springs and valve biscuits.
Mandatory-stuffing box.
It may be more profitable to buy a serviceable head.
In Ukraine, this head is worth about $ 100.
In Poland, expensive. And there is no problem with delivery.
Motor atmospheric or TD?
There is a difference in the design of the heads.