air in radiators.

air is getting into my central heating , as fast as i can bleed it out. i noticed the presssure had risen a few weeks ago, and found air in the upstairs rads. after expelling it, the pressure rose again over the next few days. i have checked everywhere for leaks, and found none. i have replaced the expansion vessels ( i know the domsetic water (( white one)) isnt connected to the heating system, but i found no pressure in it , and upon disassembly found a split diaphragm ). The red one had a small amount of water come out the shrader valve when i emptied the air from it. I took it to mean a possible split, so replaced. Air pressue in said vessel is correct, but the problem has got progressively worse each day. Now if i bleed the system,and close the valve on the last ( and furthest rad ) as soon as air stops coming out, if i give it just ten seconds, and reopen the valve , there will be no water come out again for several seconds. I havent topped up the water in the system since installing the expansion vessels several weeks ago. If i had a leak , i would surely have needed to. Its as if the system is creating the air. If the pump was faulty, would that put air into the system without water loss? I usually bleed whilst the pump is running ( as suggested by my local plumber ). If i bleed cold, then i can expel all the air, without finding more, just seconds later, but as soon as the system is turned on the air reappears immediately.. The system is unvented, and powered by air source heat pump. Any ideas welcome . cheers