1986 Moto 4 80cc rear leak
 

I'm restoring an old moto 4 80 for the kid and after getting it going and riding it around the yard a bit last night, I noticed oil seeping out of the rear brake cover. Smells like gear oil, there is no drain, fill hole, or site glass on the rear dif. I found an old thread where a guy put a 1/4 tube of grease down the breather tube to keep it lubed. Thoughts? Suggestions?