Pressure in cooling system

A mate of mine has a 300tdi which has stood idle for some 5 years. He has now been using it regularly for a few months. A few days ago he found a puddle of coolant beneath the vehicle. I replaced his water pump and p-gasket. Yesterday he called telling me that he has observed some coolant leaks from the areas surrounding the hose clips and that there was pressure in the coolant, 2 hours after the engine had been turned off. I am suspecting HGF. He is a bit more hopeful and is blaming the radiator cap. Any suggestions?

could be airlock from when the coolant went low when it leaked.
try lifting the exp bottle up above the head as the full level sits below the top of the head when bottle is in place.

if problem persists its prob HG.