Low coolant level 1995 Autosleeper Trident

I hate to say it, but generally this is likely to be head gasket failure. The excessive pressure in the tank is probably caused by exhaust gases getting into the coolant system. The diagnosis is not a certain one, but you can now buy "sniff test" kits which can detect the presence of exhaust gases in the coolant, and that should be pretty conclusive.

If you haven't already, I would recommend you have a good read of the Engine section of the UK VWT4forum, as there will have been posts on this issue there in the past. You can of course ask this question there, though I suspect that reading old posts about similar problems will tell you what you need to know. Good luck - hope it's actually something simpler/cheaper!

This happened to my van, but it is a Boxer not VW.
No loss of power or any other symptom, just disappearing coolant with no leaks. I changed the cap too, but to no avail. 
We were in Shetland at the time so there was not a lot I could do about it - tried Barrs Leaks & K Seal but neither fixed it.

Number one [or 4, depending on your point of view] piston suffered some scoring of the skirt due to the lubrication being washed away on that cylinder and although there is no lack of power, that piston now 'slaps' quite noisily when cold.

Frank

I've read on various T4 forums etc, that one cause for this is the plastic expansion tank itself and that replacement cures the problems.
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No guarantees, but at £14 it may be worth a try especially as you appear to have coolant around the seam of the tank.

Just reread your post and you have already changed the sphere sorry, I should read more carefully in future

Did you replace the cap at the same time?