It seems to act as a release valve for when the vacuum cleaner is blocked so that the pressure or temperature doesn't get too high and blow itself up. Mine used to open when it got caught on something that it couldn't suck up, like the curtains. Unfortunately it has now started to open permenantly suggesting a blockage but I have opened every pipe and there isn't one. Very frustarting, if anyone can help it would be appreciated.
Simon Gordon, June 2008
