Baxi WM 38 3RS Combi Boiler Problem

LO!
I've got a BAXI 38/3RS combi boiler and have got a leak!
On the heat exchanger I've got three valves. One for inlet, one for the heated-water outlet and a third (vent/bleed?!?!?).
Well it's this third one that's leaking (bleed/vent or whatever the devil it is). It's leaking from the middle of the retaining bolt. On removal their is a spring (which pings out!) and the water comes a gushing out. Well I assume it's going to empty all of the upstairs radiators if I leave it off long enough.
I've tightened the bolt as well as I could, but I *think* it's something to do with the spring, and whatever it should hold closed.
Well to fix the leak, do I:-
1 - have to completely drain the system? Heard you can freeze (literally with some wizz bang gadget) the joint to prevent having to do this.
2 - Is this problem likely a faulty part on this bleed valve thingy, or suddenly the pressure too high?
3 - If the problem is more 'sinister' (e.g. high water pressure), any quick clues where to start investigating?
4 - What the devil is this valve for? I've just 'guessed' it's a pressure release valve (because of the spring, and that the valve isn't just capped off).
TIA. M.
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Well, my Baxi WM 532 RS's heat exchanger has 3 'pipes' coming out of it. One is the inlet and 2 are outlets. After having received the service manual from Baxi (VERY pleased - just sent them an email asking if they had a copy and included my address and about 3 days later a nice new photocopy arrived free of charge!) it says that one is for pumped and the other is for gravity. They do actually specify which is for gravity - however mine's plumbed in the opposite way and appears to work.
So - quite possibly that's what the 3rd 'pipe' is for. Mine's just capped with a drain cock on it.
Hope that helps a little
D
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