vokera 16e loosing pressure?

my vokera 16e is loosing pressure after central heating goes off.not sure if its safety valve or expansion vesssle.i have let system go cold topped pressure up to 1 bar . turned on heating after about 20 minutes pressure was up to 3bar release valve was dripping .turned off heating pressure has dropped to 0 again when cooled down.before i topped up system i depressed valve on top of expansion vesssle air came out i assume thats correct,any help would be great.