Vent spits out water! still working on it....

JAMES MEDINA

Ok. its a pipe that stems from where the return lines meet (close to the boiler, a little above floor level). It rises from there and then goes to another part of the Building to heat other radiators.
Well this pipe has a little valve(mushroom looking thing, is it even a vent?)
on top of it (on its way up from the return lines). Its actually by the elbow where then the pipe goes on to heat other radiators on 1st and 2nd floor.
What type of Vent should I use???
Also is this mushroom thing supposed to spit bits of water?????