your press relief valve is passing, it is a safety valve that opens if the press' in the boiler goes above 3. some john wayne engineers open it to empty the boiler of water, which can cause debri/sludge in your system to to stop it shutting fully again. so once your system is filled back up it drips like a faulty tap washer and the press in the boiler goes down again. might have to get local john wayne back to fit new PRV
PAPMCG, March 2009