I ran in to the same thing. I changed that first O ring but it still leaked. When I bought the outer O ring they told me you needed special tools to get the impeller off without damaging it to get at the back shaft O ring. I couldn't figure out how to get it off myself so I let them change it. It was like $6.50 for the shaft O ring and $35 for 1 hour of labor, but it doesn't leak anymore.

Probably not what you wanted to hear, but it might help.