DerbyBorn wrote :
So - air comes out and then no water? There must be some presure pushing
out the air - but the presure goes when the water level get to the valve?
Very odd. Is the system pressurised properly? Water should **** out.Are
both valves open (not necessarily fully)?

That air is probably the gas resulting from the products of oxidation
(rust) inside the rad. There is obviously a blockage to both sides of
the rad, or the water pressure is inadequate.