I have a bottom round style diffuser with the rubber membrane. I pulled it up last week and cleaned the top off as it was not working. Put the pump back on and still no bubbles. Took apart the air line where it meets the weighted line on the edge of the bank and found out I had water in that line and I probably do in the weighted line as I got all the water out of the land line. Is there some kind of check valve in side the diffuser that needs to be fixed and can I just blow out the line with higher air compression or do I need to take the line out of the pond and drain it ?

On the Vertex diffusers, the rubber membrane itself is the check valve.

Are you sure your compressor is pumping air?

I had this happen last year. Somehow, the under water line filled on one of my three diffusers. My system was cycling to only run at night, so it must have filled overnight. That amount of water was enough to keep the pump from clearing that line while the other diffusers where bubbling more than usual. I ended up pulling that diffuser and line to empty the water out. After putting it back in the pond...all has been good ever since even with the system off through the winter.

I suppose you could use a higher pressure air line to clear the water out of the flooded line. Certainly worth a try, but make sure you ease into the pressure with a regulator so that you do not exceed the systems max pressure capacity and bust something.

Yes it was pumping air . That was on to clear the line above the water which I let run for 8 hours until I did not see any more droplets coming out of the ground line.

Maybe I will try at 30 LBs and see what that does. I was doing the same as you, on at night off during the day unless it was below freezing. Seems like late winter was when I noticed the pressure was getting lower. Must of filled up with condensate. I think when I get it cleared I will look into insulating the exposed parts of the ground line.

30 psi does not sound excessive for the line plumbing portions of commons system, but I'm not sure what kind of pressures the diffusers can take. I would still start low and work up to it. There's no reason to use more pressure than necessary.

I just had a thought. Pull the diffuser and base up out of the pond. Have someone on shore when you do this. Unscrew the diffuser membrane from the base and with it pointed away from you have the person on shore turn on the compressor. If no air is coming out you know the line is the problem. I DID have one Vertex diffuser that had the check valve stick internally and it wouldn't let out more than a few small bubbles.

What brand diffuser?

Another time the diffuser quit making bubbles on a different system. The owner put a boat hoist over the line and it sunk into the bottom of the lake, effectively pinching the line shut. I just cut the line at the hoist and ran a new piece of line to the airstation.