Always bleeding radiators?

Another possibility is... pump too fast and pulling air back down the vent, or low level water in the header tank, adjust ball valve to set water level to about 50mm below overflow.
Try slowing the pump with the control on the pump

Pete.
January 2007

If your system is open vented then the most likely cause is a blocked cold feed or vent causing the system to draw air in through the open vent other causes could be a failed cylinder coil. Also bear in mind the possibilty than it is not in fact air in your rads but hydrogen gas, easy to check just try setting light to a small amount of it as you bleed a rad. (Don't worry you won't blow yourself up).
Hope this helps.

Christian.
January 2007