Leaking 904

How many miles on it? Has it ever been rebuilt or is it original from 1974? If it's original from 74 that means it's almost 40 yrs. old. Even if it works good I'd at least do a basic rebuild. I can guarantee you all the seals are hard as a rock.

X2 There's no guessing what blend of fluids is in there today either.
A basic service will only add to that cocktail.