Our bedroom rad in a newly plumbed system has loads of cold spots - the tallest highest rad in house, so presumed air but when I bleed only a small amount of air comes out (re: my post a couple of days ago). Can sometimes hear water moving inside, almost like slight squelching sound. Managed to get a fair bit of air out last night (15º and baby is in that rm too!) Have tried to open the tap to vessel but cos pressure now so low, that if I open the bleed nut, not even water comes out!... but pressure won't rise at all, as if no water's getting ion, but ball valve on main is definitely open. Turned pressure relief valve and water comes shooting out, any ideas???