Does it have a breather plug? if so.. that's how water is getting in.
Also.. if it's so hard to service.. I think I'd pry that cover off one more time and add a drain plug to it.. shouldn't take too long if you have a drill, welder/tap/die set.
guess there is no chance of making one of the lower bolt holes 'non-blind' ?? ( ie.. drill thru the casting into the end of one of the lower holes.. that way you can just remove that bolt to drain.. etc. .. will always have to pucky that bolt.. etc...
soundguy