You have a vacuum leak. The system is stopped (except in defrost) untill vacuum exists by a master vacuum switch. That is the easy answer.

Since there is about a billion ways you could be losing the vacuum, throwing parts at it may never succeed. You need a plan and information. Start with a vacuum schematic and identify the switch I'm talking about and the vacuum to it. 78 models had the whole vacuum harness laid under the battery and the tubes are made from baking soda, they go poof is an acidic atmosphere, great planning.

The main vacuum and first place to check would be the thermo vacuum switch at the base of the servo. If there isn't vacuum there you have a place to start. One piece of info I can tell you is that the push button are almost never the problem. The only time I have seen them bad was after battery acid was sucked into them. Wasn't actually the pushbuttons, but the push on connector and the steel inserts the tubes hooked to.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician