WHat causes pressure in the valve cover?

[ggthack@swansea.ac.uk (Graham Thackrah)]

I spent the yesterday swapping the head gasket on a friends 80 sport as he has
been suffering from what we thought was a dodgy one. Water behind the oil
filler cap, poor performance and strangely, when you pull the oil filler cap off
pressure behind it.

Well we finished the job at half one last night and got it started fine. Trouble
is the pressure behind the oil filler cap is still there.

Why?

We thought of a poorly seated valve but wouldn't that just leak into whatever
port (inlet/exhaust)? Compression is nearly as good as new, high 170 lb ft,
no more than 10 lb ft between adjoining cylinders. Which is quite surprising as
the car has 120k miles on it and is a B reg (1985). Besides which the valves 
looked in much better condition than mine were when I had my head reconditioned
a couple of months ago.

On removing the head we checked as well as we could for warping but not only 
found none but the old gasket looked fine. If it had gone would it have been 
obvious? The pressure when you take the oil filler cap off is as bad now as it
was before. You can feel it from four or five inches above the valve cover and
it nearly blows the filler cap off if you leave it loosely in place.

Any help would be very much appreciated.

Graham.

P.S. I would have taken the head to be reconditioned whilst it was off but my 
friend is on a _tight_ budget.