how can i fix my vaillant boiler ?

can you help me i do not know the exact model no. of my boiler but on the labels it says VCW GB 221,240,280H.It has been losing water pressure.when the heating is on the pressure goes up to 3bar approx. however when the heating switches of the needle on the gauge starts to come back down, to less than 1 bar in approx. 3hrs. with the heating off the pressure still comes down but takes longer.Can you please advise and is this dangerous.Also does anyone know how old this boiler is and are spares easily available.when i moved in to my home in 1998 i was told the boiler was only 8 months old