Rear end

So it sounds like he filled it through the vent hole. Opening and draining via the plug should get you to the proper level...for now. If it is leaking out the axles and fouling the brakes, even when over filled, it sounds like you have bad axle seals. I would pull the axles and the the seals and bearings replaced. The will require a press, so you will likely have to take the axles to a shop to have them do that for you.

To "clean the brakes" you will need to buy new brake shoes. It will be a waste of money to do that without fixing the leak as they will foul again in short order.