are u in a modern home with stud walls? my son is, with a powermax 140 open vented, and has a drain valve on every downstairs rad. so its necessary to drain every valve.
also did u drain the two pumps on the boiler, the heat exchanger pump and ch pump.
take out the centre disc, thesize of a ten p piece, with a towel ready to catch cos its hot and also u might drop it down one of those fiddly little holes where its difficult to retrieve it.this is while u have called for heat so the pump is running.
i have done it so often that i can balance the disc on a large screwdriver to insert it.
this all takes some time, its not a five minute job and if u lose enough water u might consider topping up your inhibitor
good luck