Problems with used water softner

I wanted to buy a Kenmore water softner, but ended up getting a used one from Culligan against my better judgement. My brother also lives there and he was adament about getting a Culligan brand for their good reputation. We could not afford new, so got a used one.
Their machine never did work properly. We had them in monthly at least from the starting point. They would go down for about 5-10 minutes, then it was fixed for about another 2-4 weeks. Now that the warranty is up, they are charging me 130$ + taxes to come do their 5-10 minute whatever it is they do. They told me they had to fix a piston, or reset the backwash, alternating the story every 2nd time they come. They replaced the iron out as faulty at some point.
I think I'm getting ripped off here, and don't think there is anything legal I can do rectify the situation as it is a "used" unit. I was hoping to get some opinions on that(even some links to info or books), and if not, then maybe some good ways to get even with companies that do this sort of thing. I think Culligan IS a good company, but this particular branch is not doing their name justice.
Thank you,
Zack Johnson

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