Cooling system issues

[rockbadger]

My Mk2 Golf gti has been leaking water from the bit in the middle of the expansion tank for yonks.

Tried everything I can do, various different exp. tank caps, had a new thermostat and water pump mid last year, put a new heater matrix in a few weeks ago, and just yesterday replaced every water pipe and flange.
Now I'm stumped and getting fed up with it! Costing loads in G13 if nothing else...
Somebody said it could be the head gasket, but wouldn't that milky oil filler cap?
Any better ideas?

Thanks in advance!

[rockbadger]

Also, the little white valve just next to the coil is hissing like mad.
Have I just never noticed it before, or is that something to worry about?

[Gambit]

is it leaking around the sensor? just get some Pft tape around the threads of sensor & refit

nah headgasket would have all milky crap in the oil & around dipstick. coolant would also be oily etc

[Mr Blue]

Also, the little white valve just next to the coil is hissing like mad.
Have I just never noticed it before, or is that something to worry about?

That is linked to the brake servo which needs vacume. That shoud not have any leaks. I have seen some people use silicone sealant on them without problems but i've always replaced fauly ones.

[rockbadger]

is it leaking around the sensor? just get some Pft tape around the threads of sensor & refit

nah headgasket would have all milky crap in the oil & around dipstick. coolant would also be oily etc

Yeah it was leaking around the sensor, but I just took it for a 30 minute spin, sitting in the traffic for about 10 mins and it looks pretty dry! Got some minor leakage from somewhere under the radiator, but it seems to be lessening. Prob shove some PTFE round it anyway as you say. If it's fixed I'll be well chuffed 
That metal pipe was a barsteward, especially with the oil filter still on!

Also, the little white valve just next to the coil is hissing like mad.
Have I just never noticed it before, or is that something to worry about?

That is linked to the brake servo which needs vacume. That shoud not have any leaks. I have seen some people use silicone sealant on them without problems but i've always reaplaced fauly ones.

Don't know about that. At first if I put my thumb over it, the revs dropped.... But if I block it now it doesn't. Could it cause issues if left? I have to drive to Stansted tomorrow (1.5hrs)
Cheers!