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I am puzzled by your incident. I'm probably not going to be very helpful, but let me share what views I have.
1. The infrequent gear problems I've had have all been on my 1973 due to a weak battery, and doesn't apply to your model*.
2. It seems that your resetting of the breaker resulted in normal operation of the hand-pump process -- as it should -- but your message suggests that after resetting you were not able to repeat the failure (upon retarding the throttle after runup the circuit breaker did not trip). That's a mystery but it may offer a clue. You don't say where you are in California, but could temperature (warmer on the second runup) or higher battery voltage (from the alternators having more fully recharged the battery) been the reason?
3. One intriguing point is that the failure was accompanied by the gear warning horn sounding. That's supposed to happen only when the system senses that the gear is up when you retard the throttle. Again a mystery that might offer a clue.
4. Another clue for your troubleshooting is that the breaker tripped. That only happens when the current is too high (and a short would cause it or when battery voltage is too low, as in the footnote below) or when a breaker is faulty. I say faulty because with age some breakers can deteriorate and trip at a current less than specified; if your runup was soon after starting the engines and/or it was cold and/or the airplane hadn't been used for a while (so voltage was lower, requiring more current), and the breaker has deteriorated, that could explain it. But faulty breakers are infrequent, so I'd put this at the bottom of the list.
5. I'm always suspicious that anomalies happening after service may be the result of a problem caused by the service (e.g., a mistake or damage done to wiring while servicing), but you've flown 75 hours since servicing the gear, so I don't think that's the problem.
6. Therefore, your plans and the suggestions of skymstr02 seem like the right approach.
Sorry, I couldn't be more helpful.
Ernie
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* This is for those with newer models. My 73 has an electric powerpack, not an engine driven hydraulic pump like Mark's 65 and my old 69, and draws considerable power. A weak battery, especially if it's hot, can cause the sequence to stop in midstream when the alternators alone can't meet the power demand and the breaker trips. This is especially true upon landing, when your alternators are putting out less juice due to lower engine RPMs and the battery is hot; my solution has been to service the battery every 3 months (refill the cells and trickle charge) and to momentarily increase MP and RPM to 25/25 on the approach during the gear-lowering cycle.
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