No heat in one zone (boiler help)

[Josh Dillingham]

Hi BP, I'm having an issue with my boiler and thought I would ask here before calling the repair guy. I have a 5 zone boiler, system 2000. 4 zones are working perfectly, one zone has no heat at all. The thermostat is working and the boiler is calling for heat. The outgoing pipe from the boiler gets hot for about 20 feet, then it is cold by the time the pipe makes a 90 degree vertical turn and rises from the basement to the second floor. I've checked all valves which are all open and the zone valve seems to be functioning correctly. I've attempted to bleed air out of the pipe by using the spigot located on the return pipe for that zone right at the boiler. Water runs for about 10 seconds then the flow slows to a trickle. Any ideas?