got a GM engine in it?
if so the best i can tell is that this system is fed by a return line from the steering orbital valve going to the bellhousing, the 'scavenger tube' as he put it feeds back to tank. It circulates hydraulic fluid through the bell housing to keep the pump drive chain wet for lubrication.

As for how much fluid is supposed to be in there? Well it would only stand to reason that it will maintain a certain level if the return line is functioning properly, what that level is i do not see any references to this. If they hydraulic pump happens to be leaking into the bellhousing i don't see this as any problem other than it may cause some pressure build up which might be the cause of the starter seals being blown out? causing the leak?

Not really sure how to test for this though unless you have a way to remove a bolt or some other plug gaining a clear access to check internal pressure while the engine is running and hot to see if there is much pressure inside the bellhousing after it runs a while. I'm sure there will be some pressure but how much is the real question.
If the pump is leaking bad enough it could fill the bellhousing up too quickly and the scavenger tube would not be able to keep up with the amount and allow the compartment to fill up too much.