2007 Nissan quashquai

1.6 petrol nissan quashquai. Bought this car not long ago with a leaking coolant tank, had the water tank changed but still loosing coolant slowly and pressurising, the cooling system has been drained and refilled incase of air lock, a headgasket dye test has been carried out at the garage and didn't change colour so needs further investigation, I See more