Check gas cap Light Only With Less Than 3/4 Fuel
 

My 2003 deville only has check gas cap light when i have less than 3/4 fuel level. I tried new cap from Auto Zone did not help. Now I have new cap from GM Direct does not help. The last couple moths I refill before the tank level gets below 3/4 and the light never comes on. As soon as I let the level get below that the light pops on. Is the problem around the tank or the emissions cannister/vacuum line?