I have already changed the fuel pump and sending unit and still the gauge reads full all of the time any suggestions?

It could be that the lever in the tank is stuck or an issue in the dashboard.

Make sure you have a good ground to the tank if not make one

When you disconnect the sending unit for a while, I'll bet it doesn't read empty.

The only gas gage I worked on had a gizmo under the dash that pulsed a voltage which was converted to a current and measured by the gage. The gage was oil damped, so it didn't see the pulses. The method kept power dissapation to a minimum.