Have a 2000 s-10/ 2.2 litre. Just purchased, all hoses and radiator are new, t-stat is new. Have flushed the system including heater core.The coolant level keeps dropping to the point of loosing heat. No leaks, and pressure test shows ok, so head gasket shows ok. No coolant in the oil, and oil level is fine. Any suggestions would be helpful.

 

If I'm reading your post correctly than it's probably your radiator cap. I had a similar problem on my '96 I was losing coolant but all pressure tests showed everything working properly. Your cap is letting all the coolant flow into the overflow, overfill it than when it cools down it pulls in all the remaining coolant along with air causing an air pocket to form.

 

Although the level in the overflow never changes, and no signs of coolant loss there ( wetness, smell, etc) I'll give it a try. That would be a great cheap fix.