1986 Gas Tank Sending Unit

[11555wally]

I've recently purchased a 86 SB C10. When I picked it up it showed it had 1/2 tank of gas in it. When I got home after 60 miles of driving it was on empty. I put 5 gallons of gas in it and it didn't raise the needle hardly at all. What would be the likely culprit? I've read a few posts on testing these with a multi-meter which I don't have but I was wondering if there is somewhere I could take it to be tested, or does it have to be on the vehicle to be tested. I have the bed off and plan on taking the gas tank down for cleaning and painting. I suppose I could buy a cheap multi-meter and test it myself if I knew how. Thanks for any suggestions