I had something similar the other day. I had some methanol delivered and left it outside for the night. In the morning I wheeled one drum, complete with a coating of frost into the workshop so I could make some bio.
No matter how hard I tried, I could not undo the 2" or 3/4" bung on the drum. I tried a basin wrench, piece of steel with a couple of bolts, all sorts of things that normally work fine.
In the end and a bit short of time, I took to drilling a small hole through the centre of the bung (slowly and rather nervously with plenty of oil on the bit I hasten to add - hoping that the oil would keep temps down and there would not be much in the way of Oxygen inside the drum with the methanol).
As soon as the bit was through, there was a dramatic flow of air through the hole into the drum. Once that stopped I was able to undo the bung as normal.
I can only presume that they were filled a somewhat higher temperature than I was trying to open them at and the resulting cooling of the vapour had caused a pressure drop.
