Hello, I do not know the exact stack arrangement but for what it is 
worth, on stacks with vertical valves, I have had several with inside 
seats leaking. 

It was usually due to the leather resting on the seat for decades and 
the acids from the leather causing corrosion and pitting of the seats.  
Perhaps that is your problem but perhaps not.

Regards,
Bernt Damm
Sydney, Australia