CT, the gas check method is very subjective, and often gives a false no fault return.
An old, easy DIY and very reliable trick I use to check head gaskets, is to force compressed air into the cylinder while the engine is at TDC firing stroke. So piston TDC and all valves closed.
If the gasket is blown, even very slightly, air will be forced past the "blow" into the cooling system, displacing a bit of coolant. If the head gasket is blown, coolant will be forced out at the radiator / filler neck. If the "blow" is very slight, the level at the radiator neck will rise / overflow very slowly, leave the pressure on the cylinder for a few minutes, to be sure.
It can be a bit tricky to get it exactly TDC, so try to allow the compressed air into the cylinder gradually, as if the engine is not 100% TDC the compressed air will spin it. Don't leave the tool used to turn the engine on the bolt.....
Fill the radiator to the brim, its best if the system is bled properly as well.
I use a made up pipe with a modified compression tester fitting to force 8 bar into the cylinder.
If the engine is hot when doing the test, the level may drop slightly as it cools.
Hope this make sense...
