Raise the front of the car as high as you can with a trolley jack or just run the front up ramps, turn the internal heater controls to HOT full-on and remove the header reserve cap and start the engine. As the engine warms start squeezing the top hose to force water about in the circuit. Continue this for about 10 mins, fit the header cap, drive off ramps or lower car and try that. Should work great!
Good luck Steve
SB, November 2006