When I first start the truck, I'm running 5.5psi fuel pressure with a half-full fuel filter.
Half full filters are common. Almost to the point of being "perfectly normal" as well.
Anyway, after 10 to 15 minutes I lose ALL fuel in my fuel filter but keep fuel pressure. The truck does stay running. Is this normal?
Yes.
I've run the truck from a gas can to bypass both fuel tanks and I have the same issue.
My take on it is that you do not have an issue. Based on just the fact that it stays running, you still have fuel.
The inconsistency is the float opening and closing the needle and seat valve changes the fuel flow and demand characteristics. I admit that I never actually fully ran out of fuel in the filters like you're saying, but it gets pretty low on a pretty regular basis. Just the nature of the beast.
As long as it's running and not starving for fuel, I don't think you have an actual problem.
But still do what you're planning and what Rustytruck said. You can actually start at the top by disconnecting it from the carb and extending the hose upward vertically, then crank the engine for a second or so to get fuel back into the filter. Then re-connect to the carb and make sure it's tight. See how that purging works with the system.
My guess though is that eventually it will go back to status quo and you will continue to see the ebb and flow of gas in and out of the filter.
What is the orientation of the filter and rest of the line? Is the filter vertical, horizontal, or something in between? What's the rest of the fuel system like?
You always want to be sure that the hoses and lines are in good shape and not leaking (air can get in sometimes without fuel getting out) either inward or outward. If it looks old, just replace it because it's cheap insurance.
If you are still running the original hard plastic line along the frame, inspect it. By now it's probably going to last forever. Some few have had aging issues with the original lines, but most of those stories are from 20 to 30 years ago or so now. Guessing that if you did not have problems back then, your Bronco got one of the good batches of plastic and it's still in good shape.
But don't assume... Double check yourself.
Any rubber that's questionable, replace it. Again, cheap insurance.
Got pics of your fuel system at the engine?
paul
