Water leaking from the air automatic vent on boiler?

Hi , i woke up one morning to notice my water and heating would not heat..Checking my boiler i soon notices water dripping down from the top of it, not only that there was no pressure at all an dthe boiler would not light...On closer inspection there was a red screw thing at the top which the water was coming fro it, Looking in the Ravenheat manual on my boiler it stated this was the air automatic vent valve. Anyway way if if i let water in to the system increasing the pressure I am able to get the water to heat but then more water starts leaking from the automatic air vent...I'm guessing i just need to replace this part? or is there something else going?