I have a 05 Duramax and having problems the coolent system. After a while the coolent gets low when i go to and fluid to the system to fills back up but keeps going on. have had a lot of presure test done and they tell its fine. Then tried to tell me that it was just an air pocket. now they r trying to tell me that its blown head gaskets. Does any one have any ideas what it could be I really need some feed back need all the help I can get. Thanks