Morco super compact FEB 24ED low water pressure due to leaking from overflo?

I have morco FEB 24 ED in my caravan I rent it out and found the water top up valve open and pressure on 6 bar, the water top up valve I turned off . next day I had F3 on display low water pressure and o bar on pressure gauge ,, the water had ran out the overflow pipe , I topped it back upto 1.5 bar same thing happened again , is a=it a faulty regulator valve or expansion tank that could be cause ?