Mercedes poor quality service

Mercedes C220 cdi had waterpump failure at 60 312 km which was replaced by Mercedes in June 2013. At about 88000 km new owner noticed slow coolant loss. Mercedes diagnosed water pump failure again, less than 30 000km after previous repair and insists on owner paying 40% as per service plan or they will not complete the work. This is clearly not a service plan issue - this was either due to another faulty part or poor workmanship and Mercedes should, without trying to pass the buck and aggravating the customer, replace the item free of charge. I own 3 Mercedes cars and will not consider buying another if this is the way Mercedes now operates.