GMC Repair: fuel gauge erratic reading, chevy s10, gauge works

Question
I recently purchased a 2001 Chevy S10 1500 LS, V6, 4.3L 4x4 with 117,000 miles at a used car dealer. On the test drive I noticed the gas gauge which was at 1/4 tank, dropped to empty and the low fuel light came on, it later went back to 1/4. I had the dealer replace the fuel sending unit. (At least they say they replaced it and the float, I don't trust them and should have asked for the old parts.) The problem never went away. Gauge works fine from full down to 1/4, but at 1/4 it will always drop. If you turn the key off then on the gauge immediately will go up to 1/4 tank after having read empty.
The truck also seems to crank longer than usually much of the time, and I have been told by an independent mechanic that the check valve in the fuel pump is bad causing fuel to drain back to the tank. I was going to replace it myself but wondering whether to buy just the pump or the fuel pump module which is twice as much. I don't know if the fuel pump would have anything to do with the gauge problem.

Answer
The fuel pump will have something to do with the fuel gauge issue. The fuel pump could have the fuel sending unit integrated into it. At times, even though the part is new, it can still be defective. This is something that cannot be completely cured in mass production. I usually see this in electrical parts like coils, coil packs, alternators, and starters.

 Unfortunately, if you have one of the fuel pumps with the sending unit built into it, this could be the case. I would take it back to the dealership, and have them replace it once more. I am not a large supporter of recommending dealerships other than vehicles under warranty. That is what I would look at and try. As for the cranking, I would like you to try a couple of things.

 First, go to any mass retailer and purchase Lucas brand fuel injection cleaner/fuel additive. I use this myself in my 1999 Dodge Ram and 2001 Kia Rio. I have gotten good result, very good actually. Lucas is just about the best, in my opinion, you can buy. The other thing is to change your fuel filter. I run Lucas in my transmission as well. I do this every six thousand miles or every other oil change.

 If you still have problem email me directly. marvin.saunders69@yahoo.com I did look up the part you had replaced. There are three electrical connections and a float. Power to the fuel pump, power from the sending unit to the gauge, and the other I am not sure of and would have to do further research. I will say that you have a defective part. Demand to have this replaced if you get "lip service" from your dealership.