My 78 originally had AC, but the pump no longer functions, and I have a ton of vacuum hoses that need replacing. Is there a way to simplify the system, can I remove or bypass the two magnet valves or eliminate the fast idle actuator ect.

From what I recall (I've been away from the Datsun AC systems for almost 2 years now) one of the magnet valves is for the FICD and the other is for the mode doors. If you don't plan to have the AC you can bypass the one for the FICD but you'll need the other one for heat, defrost, etc.

understood. That would simplify things a little. What would occur if the other magnet valve was bypassed meaning constant vacuum to the heat controls? or the vacuum tank for that matter. Is the vacuum tanks propose to maintain steady vacuum?

Edited February 2, 201114 yr by grantf

That's pretty much what the tank is for. It stores vacuum to help keep the vacuum supply constant during the different engine speeds and loads. I'm not sure that the mode door actuator would work properly with constant vacuum since the magnet valve opens and closes based on the selector position.