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...It's purpose is to prevent vapor lock due to "Heat Soak", which can cause fuel to boil in the fuel rail.
It isn't meant to hold pressure overnight — the spec is to maintain at least 1/2 pressure after 20 minutes.
As a diagnostic test, you could force the pumps to run for a few seconds the next time you have a hot/warm start situation. Do this by Jumpering the Left/input side of fuse 4 from the (always hot) Left side of any fuse 6—10. The recirculating fuel may clear any vapor from the fuel rail.
The +12V at input to fuse 4 goes two ways:
1) Thru the fuse to the Tank pump
2) Directly to the Main pump
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Bruce Young '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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