The problem is most likely to be occurring because the air separator (the correct term for the device you describe) is positioned in the system close downstream from the pump. It should placed upstream from the pump, between the boiler and pump such that water passing through the boiler gets heated, passes through the device, then onto the pump and hence around the system. What is happening is that the positive pressure of the pump is overcoming the 'head' of water created by the expansion tank. Bear in mind that this positive 'head' of water pressure is only the difference between the level of the water in the expansion tank and the top of the loop that the expansion pipe rises to (maybe 60cm?) above the expansion tank. Since one bar of pressure is equivalent to 10m of water 'head', you can see that the water will very quickly get pumped around the expansion system instead of being pushed around the heating circuit. The elegant solution is to move the relative positions of the 2 devices but this may not be easy. An alternative is to raise the height of the loop above the expansion tank to create a better head. You could do thid using the modern plastic pipe, cutting the existing expansion pipe, using a straight coupling to move into plastic pipe and taking a huge loop right up to the apex of the roof, returning to a few cm above the height of the tank. Even this may not solve the problem.