It probably has to do with the following: The one way valve is leaking. It is probably located in the carbeurator. It is allowing the float bowl to empty back into the tank down the fuel line.
If it is a 350 with a four barell carburator, the fuel filter is a cartridge inline at the carb. Unscrew the large nut where the fuel line goes into the carb and the fuel filter is inside.
its in the carb
in the carb inlet
1985 CHEVY PICK-UP IS CARBURATED... THE FUEL FILTER IS LOCATED IN THE CARB.. BETWEEN THE FUEL FEED PIPE AND CARB. IT'S A PAPER TUBE STYE FILTER. GM# 7028281
1985 Chevy fuel filter should be where the fuel line screws into the carb. use a 1 inch wrench and unscrew the nut that holds the line to the carb pull out line should be a small filter ,spring and washer
the fuel filter is on the carb. you have to take off the fuel line
on the fuel line running from the tank to the carb.
if it's a four barrel, it's in the carb.
Disconnect the fuel line from the carb with a 5/8" line wrench then unthread the fitting in the carb with a 1" wrench. The fuel filter is inside that fitting. Take note how the filter and spring are assembled into the carb for reassembly.
It's in the carburetor, take the fuel line to the carb off and then the big nut on the carb and the fuel filter is behind the big nut.
at end of hose coming from throttle body,or carb.
in the fuel inlet on the carb.