I have a Mercedes Benz ML350 that had a bad power steering pump which i had a mechanic replace 5 months ago.The new pump started going bad too,which is unusual but it had warranty for a year.I recently went back to the mechanic to replace this pump again and he wanted to charge me labor to do this.I didn't want to pay labor for the same problem twice within 5 months.Is it right for the mechanic to do this?