Please describe your boiler and system type to get a more accurate replay. 9 times out of 10 its an external control of some type that is sticking and causing the boiler/pump etc to continue.
Hold a container (small tin can?) inverted over the bleed screw and collect the gas that comes out. Still holding the container upside down, try lighting the contents.
Do it like this. Before you do, have the pumping over investigated. Usually a blockage near the position where the cold feed and vent are connected at the moment. Check around that area with a magnet to detect any magnetite build up.