If the floats are bad and gettin fuel inside them, they will not read full, as they will not float. I would be more inclined to say the circut board, or the arm that rubs against the board is bad as the OP stated that it reads ok untill it reaches half full then drops to empty.
Yeah My 85 does the same damn thing when it say's E it only take's 10 gallons to fill her back up.
My '86 IDI rear tank does this, gave me my first WHERE'S A DIESEL STATION moment as I was empty in front trying to use up what fuel was sitting in it when I bought the truck. Took 8 gallons to fill. I have since found that it shows "E" by about 100 miles, stays there to maybe 125 miles, and then comes back up to 1/4 or so and stays there until dropping back down at 200 miles.
Excellent resource, thanks so much for posting that. I can deal with my rear tank on the IDI for now but the C4 race car is darn near random so I think I will send it to them in the off-season - many thanks!
What you are describing sounds like the variable resistor part of the sender is bad. The float arm moves along the wires and it has bad spots not making good connection. Not a float problem, as it does read right when full or down a bit. The problem is the sender. A fuel gage is just a resistance gage that is calbrated E to F instead of numbers. I forget the Ford resistance values for the fuel tank sender range.
Mine is doing the same thing. Right after I fill up, the gauge reads about 3/4 full. My filler neck is botched up, so I only can fill it that much. After a while when the gauge reaches half, it drops to E but doesn't hit the peg. I knew I didn't burn that much fuel. I thought I was going to run out of fuel when it first happened. I always carry a spare can of fuel in the back just in case, so I wasn't too worried.