If its like the merc system, when you shift OUT of gear, the switch will activate, then engine dies (momentarily), the load on the gears (in the leg)
that's one way of describing it.....the switch would be better hooked up to an alarm (buzzer) so it will get your immediate attention..
a closed system should build pressure as it heats up....but shouldn't exceed ~ 7 psi or so....is the heat exchanger large enough?
How does the primer bulb do if you pump it up without hooking it to the engine?
#7 on throttle rod diagram @ boats.net?