Air vent question

[Satchmo]

Please bear with me as I am not a plumbing expert

I have an open vented, gravity feed central heating system. The system was power flushed a couple of years ago and a magnetic-trap fitted on the heating output path of the tri-valve. I manually vent a very small amount of the air from the system a couple of times a week (when the system is turned off) by bleeding the top of the magnetic-trap and by loosening the capped vent, on the other side of the tri-valve, mounted on a rising pipe next to the water tank, which takes about a second to vent (giving a small 'thump' as the water reached the top of the vent pipe).

However, last night when I tried the capped-vent, no air (or water) escaped. I was able to remove the cap fully with no effect

I checked the F&E tank in the loft and it appears to be OK and and the stop-cock is functional.

So far today the central heating (and water) seems OK with no noise and radiators on full heat (with no cold spots).

Could this be a blocked cold water feed?

Can any one advise me on how to proceed...ie. safe to leave it or get a plumber asap?

Cheers

Satchmo

Sorted if (indirectly) at lunchtime. I removed the vent cap and popped a thin wooden rod directly into the vent shaft to see what the water level was and
lo and behold water started pouring out the top of the vent...so it was a blockage in the vent itself, which has now cleared ..phew

Satchmo

 

[Renewable M]

well done grasshopper, it's often the same with the bleed pips in radiators. For which, the high tech 'darning needle' does the trick.