It loses about a quarter of its 1 bar pressure over a few weeks. Only needs.a one second burst from the filling loop to bring back to 1 bar so not much loss. No visible leaks. It's ex open vented, now sealed, with a megaflo. Rads and pipework are a mix of new rads with new plastic pipework and 40 year old copper pipe and rads. I am repiping and replacing the old stuff as I work round the house but in the meantime is it likely that I'm losing a bit from the old parts and it's evaporating off?