I have a 1978 F-250 with the 460. When I have it running sometimes the filters are nearly full, sometimes nearly empty, usually somewhere in between.

I changed the fuel pump. My next step is to look for leaks in the line. Would sucking in air cause it to only sometimes empty like that? Any other ideas what it could be?

 

Most likely caused by a leak between your tank and fuel pump. This could be anywhere from the fuel pickup(pinholes in the tube) to the rubber fuel line being nicked or deteriorating, a loose fitting or hose clamp or just plain old fuel lines.
Look along the length of the fuel line for drips.

 

I'm actually gonna have to say that what youre experiencing is totally normal and all of the carb'd trucks I've had with clear fuel filters did the same thing. A friend of mine who is probably one of the best mechanics I've ever known agrees with me. As long as its running fine, you can't really go by whether or not the fuel filter is fuel.

 

x2, the "air" is probably fuel vapor as pressure rises and drops and even as a vacuum is created

 

I have been using the clear ones on various engines and they all do it. In the past i would throw them away and get new ones, same thing. I bet the metal ones of the same design do it too , you just can't see it happen . So get one you can't see the fuel with.__

 

I don't know guys, iv always had luck with keeping mine slam full of gas.. when it isn't there is either an air leak or rust clogging the pickup in the tank.. at least that's my experience with it.. just my .02

 

Is is not running correctly or is it acting like it is fuel starved at times? Like when it under a load or when off roading? If no to those questions, then why did you change the fuel pump?

I have seen fuel filters full at times and 1/2 full, if it ain't broke don't go messing with it for no good reason.

Yes go ck all your connections and your fuel line conditions, if you find a fuel leak or it is not running right then go to fixin it.

2 cents plus a 1.00

 

Yeah these fuel pumps are nearly bulletproof

 

As others have stated, if you do not have a fuel smell or visible liquid fuel leak, don't worry about it. I and other's have troubleshot this problem several times when these Trucks were new after a owner installed a glass encased fuel filter.

Of course back then NEW was 12 months or 12K miles...

 

btw, and I should have said this, my truck has brand new aluminum fuel lines, a brand new poly tank, brand new napa rubber lines connecting the new aluminum lines to tank and pump, aluminum lines from pump to carb, and a new fuel pump. And mine is hardly ever 100% full, half the time it looks about half full. So definitely no problems with my setup. I even bought a tube flaring tool to put the right flares in the aluminum to hold the rubber on. new stainless clamps, whole deal. only original thing at this point is the carb. factory 4 bbl.

 

...or it could just be normal.

 

77&79, Under load, towing and sometimes hard accel (especially up hill) it will "bog." It feels just like it does when I need to switch between tanks. I read that acting like that could be attributed to a fuel pump going bad.

I also did dizzy, timing and adjusted the carbs (those needed to be done anyway-it was running really rough)

My next guess for these problems is the filters on the fuel sending unit.

 

Most likely the fuel pump
The screens on the fuel pickup tube are very fine mesh sometimes do plug up.
I would start by changing the fuel pump first.

 

Did fuel pump and regular filters first.

 

Right, I just reread.
Has this always done this or is it a recent development?
What carburetor are you running?
Are your gas cap vents plugged?
Has your truck sat for a long time?

I do agree from what you describe that it is a fuel delivery issue. One possible (but potentially disastrous) way to determine if the fuel pickup screens are plugged is to unhook the fuel line where it goes into the fuel pump and blow air into the line back to the tank. This will effectively backflush and temporarily clear the screen.
I don't think I would use more than 10 psi of air. 5 would probably suffice.
Be gentle, you can do damage.
Hook it back up and see if that doesn't help. Chances are it will plug back up soon but at least you know where the problem is.