Sorry, I didn't catch that the prior owner had the water pump changed. So, yes the shop owes you nothing. I wouldn't even mention it to them since you have no firm facts.

Yes, I have taken people to Small Claims, and yes it is not easy. Usually they are smart enough to change their attitude just from the threat. It only got to the court stage once, 6 months ago. It took a long time but a tree trimming company working for the power company ended up paying $1300 for damaging my trailer when they came un-announced then tried to sneak away.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans