vokera linea 24 problem?

my linea 24 boiler keeps losing pressure over approx 2/3 week cycle. when c/h is running at 60 degrees the water pressure is at 3bar, if water temp is increased the pressure goes up to 4bar, the heater keeps working i.e. does not trip out.i've checked the overflow pipe leading outside and there is no sign of water so it would appear that the pressure relief system is not opening, however when i looked up at the boiler exhaust vent i could see what looked like traces of moisture ( stains ) running down the wall from the vent. also after topping up the system, if i remove the topping up flexy pipe, water ( approx half a cupful ) runs from one of the valves ( i would assume the system side ), and the pressure guage drops to zero, i therefore have to leave the pipe in place after shutting off the water top up tap. when the boiler is producing hot water for bath etc the water pressure remains at 1bar.
any help would be greatly appreciated
G.S.