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Big Al,
I'm not expert but I'll offer you my opinion...
The power pack will pump until the pressure switch trips. It should be set for something like 1500 PSI. If it's set low, then the pump will shut off prematurely. For instance, the pressure switch may trigger at 1000 PSI - and the gear has not yet completed its operation. When you hand pump it, it only takes a couple of pumps to feel the pressure because the system is already partially pressurized.
An alternate theory could explain this symptom also. I had such a failure on my plane. During ground tests, if the mechanic pushed on the gear leg (creating resistance), the pump would die. I'm not sure I completely understand this symptom - because I would expect a weak motor to pop the circuit breaker in this case (drawing more and more current when it can't overcome the resistance), but replacing the power pack solved it.
I'd suggest you put the plane up on jacks and run it with a ground mule (which has a pressure gage in it). Measure the pressure switch in the plane; at what pressure (according to the mule) does the Cessna pressure switch trigger? If it's too low, this can be adjusted.
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