I am pretty sure the coolant level should be at the minimum when the car is cool. I personally have filled both my 944 and my former 924S to the halfway mark cool and never have problems.
I recently changed my headgasket and had rough time getting the air out also. I to bought a 8mm grease fitting and punched out the spring and ball bearing. I bought some clearn vinyl tubing from HomeDepot..but need to get something that stands up better to heat....the vinyl stuff gets real soft with the heat. I think i used 5/16th's diameter. It did work great though. Defenetley cleaner than spewing coolant all over my nice and freshly put together top end..
.