Hi all i have done a batch of brew for the first time in several years , i bought a boots pressure barell on ebay nice n cheap its got a co2 injector but i cant work out how it works, the bulb holder sits at right angle to the barrel opposite the bulb holder is a knob which pushes in on a spring, now is this knob a safety pressure release button or does it release co2 into the barrel when pushed , i realised after buying these things are as old as the hills i think its 1983, any help would be much appreciated, thanks.