loosing presure on my vokera 24?

When I fill my vokera 24 boiler to just over 1 bar and then switch it on the needle goes right to the top and discharges water out of the blow pipe then when I switch it off, the needle goes well below 1 bar and I have to keep it on around 53' --any more and I loose more water. I have checked the expansion vessle and it's ok. Thanks to Andy plumb.