One thing you might want to check for is to make sure your axle isn't bent.  Mine was leaking real bad and the cause was my axle was bent at the ball.  I've rebuilt it twice and it still leaks.  I'm in the process of replacing the axle housing, but need to get another rebuild kit from Marlin.

Notice how low the axle sits inside the ball.  It was a bish getting the axle in there when it's bent, because I have to lift it up to be able to push the axle into the housing.  By doing that, I damaged the inner seal.