I've got a '89 turbo and have never seen much pressure stay in the expansion tank for very long. After a few hours rest there's usually just a little puff, if that, when I release the tank cap. Irregardless of the ambient temp. My cap is green. The head gaskets almost always seem to let go between the cylinders where there's no water passage in the head so there's usually not too much warning. Just some sudden really rough running and engine knock followed by low compression in two adjacent cylinders. There is alot more pressure in the cylinders though than in the cooling system so I would think it would be possible for the engine to initially pressurize the cooling system with gas vapor without the lower pressure water flowing back the other way. But I would think the expansion tank cap would vent the excess pressure?? That is, if it was working correctly. Are the hoses new? I've read where they can sometimes develope an internal blockage that isn't evident from the outside.