[biketopia]

Definitely try a new GM cap, been known to cause issues on the LLY. They are also known to pop head gaskets but you might be lucky and just need a cap! The soft upper hose after sitting all night is a good sign. Get it full of coolant and a fresh cap and keep an eye on it. Also look into the mouthpiece mod, something about the turbo mouthpiece was restrictive on those and lead to issues which resulted in overheating. Putting an aftermarket one on or doing some mods to a stock LBZ mouthpiece would do the trick.