Check the fan and fan clutch too. The fan should "whoosh" when it's hot and you accelerate from idle.
Is it possible you have air in the system? I think 104's have bleed screws in the block. Also, see if you get heat from the heater core.
A symptom-free pump failure just doesn't sound right, especially when you get results with your hand test.
Here's a pretty thorough link (for Honda's) jsut to make sure you've covered all your bases. It shows a corroded impeller, but notes that this is a very extreme case.
http://www.tegger.com/hondafaq/overheating/index.html