loosing water?

my car is a citroen c5 diesel auto, i am loosing water at a rate of 1-2 litres after approx 40-50 miles of use, i have replaced ther water filler cap, because i thought that was faulty, but iam still loosing water, i have no signs of milky oil, no bubbles coming from my water header tank when i have the cap off, i have had the engine pressure tested and it is loosing pressure,, please can u help !!!!