Does it need topping up each time the system has been hot and cools down again?
If the bladder in the expansion vessel has failed then you would top the system up to operating pressure, the heating would come one and the system would weep out excess pressure through the safety valve. Then, when it cools down the pressure would drop.
Oh and that fernox leak sealer, I had a small leak under a freshly tiled floor and fixed things a treat!