oil leakage/seepage from oil filler cap

Replaced the gasket, and still leaked. Checked the CCV (disk on top of valve cover) for fear of blockage but there was no visable build up of any kind. I guess I have two possibilities:
1. does the tension on the oil cap eventually start to fail? The vw parts guy said he's never seen one fail when i was debating to buy just the gasket or the entire cap.
2. Are there any other places to check for pressure build up?

Thanks fellow tdi'ers!

Here's the easy and cheap fix

http://forums.tdiclub.com/showthread.php?t=284116

If you have a friend with a similar TDI you could swap caps to see what happens.

If it no longer leaks for you but does for him then you know it's just the cap.

Cheap and simple.

Thanks on both accounts! I'll give both ideas a try.

The dealer replaced gasket at the base of cap extension and it was still leaking so he replaced the complete oil cap and now no more leaks.
He had misplaced the extension (a old mecanic but new to VW)so the engine cover did not fit properly and oil was splashing over the valve cover.The extension must be in a certain position so it does not come out of the valve cover when you remove the oil cap only.They had to shampoo my engine and my next oil change will be free for my inconvenience.
95,000km so far and 0 problems,original tires,brakes and everything else.