Hi Patton,
Thanks for your service!
I'm a rookie Z driver so I don't have much knowledge but when you get a Helm & read about the vacuum system for the secondaries it will explain this:
The pump is only supposed to charge the system &/or maintain 12"Hg in reserve tank and goes off at 17"Hg.
When the motor fires the engine vacuum takes the place of the pump. That is why you got a good vacuum reading with the motor running and when it was off the vacuum test showed a falling reading due to a leak in the system.
The check valve RedZR is talking about is one he added as a backup/or replacement for the OE one in the system. Great idea! I looked on my parts CD and can't find the one that is supposed to be there so I think it is intergral with the vacuum hose assembly and the Helm says there is one in the pump??? I'm probably wrong but I know I looked & can't find it.
I don't know if this helps but the diagnostic chart in my 90 Helm says to use 20"Hg as a diagnostic benchmark to test system integreity.
Sorry I can't come up with a better insight but I thought this might help.
Tom
