My 1992 K1500 has a problem with the fuel level, and the guage reads about 1/4 tank low. Disconecting the sending unit allows the guage to read past full. I checked the voltage suplied to the sending unit and it is 8 volts. Is this proper? What should it be? What could be other causes of this problem?

What you need is a variable resistor that goes from 0 ohms to about 150 ohms. Hook up one end to ground and the other to the pink (maybe purple)wire at the truck side of tank connector . At around 0 ohms gage should be on or below empty, full on gage is about 90-100 ohms. Vary the resistor and watch gage. The voltage you see is what gage sends to tank unit, some are around 5 volts, some a bit higher. DO NOT hook up resistor to grey wire at connector, that'll blow fuel pump fuse. One more thing, make sure the ground for the inst. cluster is tight, it's right there at the diagnostic connector and known to get loose, also make sure tank ground is good, it's right under the tank connector itself, black wire under connector holddown