Hi mate, What you may find and I've had on afew cars , different makes and models what happens is things have hairline cracks in the overflow or radiator. What happens is when the car heats up and under pressure the cracks open up and you get small leaks from the cracks when it cools down the cracks close up and will pressure test fine. So if you see a small coolant leak somewhere just after you have driven it and it still hot it's probably needs replacing where you see the leak. Sometimes these cracks are hard to find. Good luck My 2 cents