Illegal Vette

My radiator cap is leaking coolant, and I'm wondering why. I replaced it and it made no difference, so I started it cold with my radiator cap funnel and a lot of air came out of it. Eventually though the air stopped coming out so I topped off the radiator and after driving it today it's still pushing air past the radiator cap. I'm going to do the cold start with the cap funnel on it again to see if there's still air in the system but I'm really confused that it's coming out past the cap, why wouldn't it vent past the vent port under the cap? My system is plumbed like this:



Maybe the surge tank is somehow preventing normal venting? Have any of you experienced this as the result of a blown head gasket?

yotaracer

does the system build pressure when hot? maybe tank is cracked?