You ned to replace the safety pressure release valve. I've gone thru 4 of tese on the same model boiler.
Once you turn that knob to release out pressure it never reseals again properly. It's a straight forward thing to change. One nut on the side and a clip on top. Isolate the boiler (ie switch everything off and close the pipes under the boiler) Remover the side nut. remove the clip then yank the valve out. It may need a good pull.
Replacement is just the opposite way around. But make sre your really push the new one home. If it starts leaking from the cip joint when you refill then give a good push on the valve to seat it properly.
Hope this helps
Steve, December 2008
