I haven't picked the car up yet, but assuming the leaking problem is fixed, I plan to let the original mechanic know (in a diplomatic manner) what had happened. I'm not angry - people make mistakes. But my question is this: Is it reasonable that I should expect a refund of my $150, or should I kiss it goodbye? It seems very unfair that I wasted $150 on a repair that did not work, simply because someone didn't follow the instructions.
If he refuses and makes excuses, should I drop it, or pursue legal means?
The first part of your plan sounds very good. Calmly explain the problem to the orginial mechanic and ask for your money back. Make sure you understand what when wrong and can "present a case" to the mechanic better than "you suck". If those efforts fail, I'd say chalk it up to experience, and take the loss. But as Pedro said, I'd let everybody on the planet know about your dissastifaction. If the guy gets really crappy about it, I'd report it to the BBB and any other consumer rating type group. If the mecanic is part of any sort of association, whether for mechanics, a small business or as part of an auto dealership, you may explore fileing a complaint with them.
Personally I wouldn't waste the time on "legal means" for $150. You'd have to take him to small claims court because no laywer is going to take a case over $150. My time is money so anything over 1 hour's time involved in a small claims matter would be a loss IMO. And you have to figure in driving to court, waiting for your case, time to prepair, time with the new mechanic to get written "statements" from him, etc. Ultimatley, I'd lose money trying to get back a realtivley small amount.