Baxi instant combi he loosing pressure?

We have had the esxpansion changed on our boiler as it was constantly loosing pressure and water was spilling outside. Since having this changed we are still loosing pressure only after the heating has been turned off and it's cooling, once i put the presssure back up it will stay on 1 bar rise a little when heating first comes on but when it goes off the pressure falls quite quickly to 0. We have had all the floorboards up to check for leaks and cannot find anything. any suggestions greatly appreciated thanks