Do you have a remote filler on the line going to the radiator?
In a Cobra, the radiator is typically below the thermostat, so you will never be able to get the system completely filled using the radiator cap.
Most use either an expansion tank, or remote filler like the Moroso unit that you splice into the hose...
http://store.summitracing.com/partde...5&autoview=sku
- Dan