When you've got a bit of air in a brake system that just wont shift, slipping this handy tool over the front brake lever will hold the brakje on hard and thereby effectively leave mastercylinder valve open so that any air bubble in the system, over time will find their way out again. If it's a real pig, leave it on over night and in the morning you will wake up to a nice stiff brake.

This tool is also great when transporting bikes to keep the front brake locked on.