Easher

Hi all This is my first post but have used this forum for tech info many time...It's great... I have a little problem that I cannot solve, my Y Reg 1.9 PDi galaxy is leaking from the header tank cap about 250mil everytime I drive over a 20-30 mile range. If i drive longer I get the 'STOP' message come up and i put in about 500 mil of water. I have replaced.... CAM BELT WATER PUMP THERMASTAT HEADER TANK & CAP AUXIL PUMP None of this fixed the problem. Any suggestions would great. Thanks Phil