Worcester Bosch RSF combi boiler. How do i keep the pressure up?

I have a wor/bosch combi it had a leak from a valve. Behind the electrics on the boiler. The valve had an orange knob. I tightened it up and the leak stopped but now the pressure keeps dropping. It goes from the red bar needle to zero in about 30 min. So now when we want hot water i have to open the pressure valve use the hot water and by that time the pressure has dropped again. We are doing this 5 or 6 times a day. Its a worcester/ bosch 240 RSF. Can anyone tell me what the problem is. What could have caused the leak and how do i maintain the pressure. e