my SUMMIT brand liquid filled fuel
gauge is very close to my carb inlet.
I found that as the engine heats up, so does the liquid in the gauge.
the ONLY accurate fuel pressure reading I get, is when I start the car, and the engine is still COLD.
I would have to isolate the pressure gauge from engine heat, or the fuel pressure at the gauge will drop with actual engine temperature rise.
I learned this the hard way, of course.
tell me if you find the same problem.