I don't have any printed information on this old cat, but I am old enough, and enough of a cat tech to have worked on these when they and I were young, and still [kind of] remember these units.
if the bell housing is filling up with fluid, the most likely cause is the pickup screen in the fitting the hose on bottom of the trans. (provided this really is a standard wet-clutch transmission, [there were a few transmission options, including a true hydrostatic transmission]). this screen is to the inside of the trans and will not be seen unless you take the fitting out of the trans case, that the hose is connected to. this is a very fine mesh screen, and will be where the flow (out of the bell housing) is most likely restricted.