peugeot 307.sw cooling problems?

can any one help i have a peugeot 307 sw it keeps dumping the water from exspantion bottle and has bubbles in it aswel i have blead the systom times with no joy i have had the garrage put a presher test on it for head gasket but it was fine no problems their but it keeps dumping the water out the exspantion bottle this runs empty but i can drive the car for miles heater hot no boiling but the bottles always dumps water i have even changed the bottle and cap and still it dumps the water can any one help on why it does it or how to solve it thanks roger