Classic/Antique Car Repair: Chev fuel pump, fuel pressure regulator, manifold vacuum

Question
QUESTION: Should fuel pump pressure drop when engine starts? When key is turned, pressure is about 58 lbs. It drops to about 50 when engine starts. Is that normal?
Thanks

ANSWER: Year, make, and model of the car please.

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QUESTION: It's a 2000 Chev Silverado  K15703 4X4 8 cyl 5.3 liter.
Thanks.  

Answer
The pressure will drop with the engine idling and go up when the engine is not running or you stomp on the go pedal. the fuel pressure regulator reads intake manifold vacuum to help lean out the mixture when the engine is under no load and then richen the mixture by increasing fuel pressure when load is applied.
Brad