you almost got it right...disconnect the line at the pump, plug that hole, and run it from the unloader to the tank. Try to connect to the tank away from your supply line.
Take a close look at your unloader. See if it has a bypass part to it. It may be closed off. If it is take out that block screw and screw in a hose barb then put a hose on that to a holding tank. This way when you depress your gun the unloader will switch to allow water to go into your holding tank. From the tank the water goes back into the pump.
In essence what your now doing with the unloader in bypass the water just keeps circulating through your pump then to unloader then to tank and back to pump over and over again so pump will not overheat. This stops when you squeeze gun where now water goes out of your wand and the pump as in bypass keeps circulating water so it never heats up to cause damage.
That should do it:)
