Glow worm 30cxi, leak when I turn blue knob?

Hi
I have just removed my radiators to do some decorating. When I had finished I put the radiators back on, connected them back up.
The pressure on the boiler was 5.8. I opened the bleed value on the radiator to fill it again. The pressure on the boiler dropped to 3.6. I slowly opened the blue tap at the bottom of the boiler. Water started leaking / coming from a brass, what looked like a value. This brass object was to the left and up a bit and was connected to the blue tap.
I hope this makes sense to someone
Do I need to do anything else the re-pressurise the system after filling a radiator.
Any help would be great.