Should fuel filter always be full?

I got a 79 pickup with a 400 engine. I noticed that the clear fuel filter fills with gas, then goes nearly dry and fills again (while engine is running). Is this normal? Shouldn't the filter be
constantly filled? I also have a problem where the truck acts like it's going to die from a lack of gas, particularly under load and on hills. I am wondering if the fuel pump may be weak..... ???? The carb was rebuilt about a year ago, and is tight to the block with no vacuum leaks, since I just replaced all the vacuum hoses.
Whether I rev the engine, or it's at idle, that filter tends to fill and empty regularly.
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snipped-for-privacy@nospam.com wrote in

Your truck is showing signs of a weak fuel pump. Check it out quickly, because if the diaphragm is ruptured, your crankcase could be filling with gas.
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It does not go 'dry' it is filled with gasoline and its vapors. What you have is a fuel pump that 'pulls' fuel to the carburetor, 'gases' unlike liquids compress. When the pressure drops some of the gasoline turns to a vapor within the filter housing. Newer vehicles that 'push' fuel from the tank to the injectors and are therefore not subject to vapor lock of yore.
mike hunt
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