You should bleed the rads thus:
Fill the system to about 1.5 bar
Run the heating and DHW alternatly a few times, then let the heating get to max temperature (about 80 C) for half an hour or so. (Windows open!)
Then shut down and let the system cool to almost ambient temperature.
Then bleed rads, starting downstairs and then move upstairs.
Once this is done, you should have no air being introduced. You may wish to re-bleed the upstairs rad or rads after about a week to get the remains of any air out. You should not have much, if any, though.
When you add water, you introduce more air, which is why you should initially set the pressure to about 1.5 bar, then bleeding will bring it down to about 1.0 to 1.3 bar, which is OK for most systems, without the need to add more water....and air.
If you find you are getting air still after the above, your system is drawing air in somewhere. Assuming your boiler is not the cause (and some new boilers can do this if they have O-ring misplacement - sometimes air is introduced when on DHW, which is then pushed into the heating circuit when the diverter valve moves), you will probably have some slightly weepy joints or rad valves that pass a little air - fairly common with today's cheap fittings. The best cure for this is to add Fernox Leak Seal (about £10 per litre) and let it search out the weeps and air inlets - they are only tiny and the Seal will not affect your boiler at all. Or spend ages checking and re-doing every joint and valve.......
Good Luck
