home heating boiler SIME Super 4?

The DHW went very dirty and so I reduced the water pressure setting when cold to 1-1.2 bar on the boiler as recommended in manual. The water discolouration eventually ran clean but then later showed some discolouration but is now clear.
However the pressure has decayed and eventually the boiler tripped with red light on.
I introduced water to boiler and pressure rose then boiler started and ran ok.
But the boiler water pressure indicated now falls over a period of 24-36 hours to 0.6 barg.
I would like to know if there is a mechanical problem with the boiler that can be fixed or is it something else.