Air Lock in Kitchen Sink Drain?

[marant]

Here is the scenario:

1. Filled fresh water tank with water and one cup of bleach to sanitize. Ran water through all lines with drains open.
2. After a day or so turned on the pump to run all of the fresh water out of the tank into the grey tank to both somewhat sanitize the grey tank and drain grey tank. Grey tank empty when started.
3. While filling the grey tank (toward the end, probably only 4-5 gallons left) the kitchen sink stopped draining, filled 1/2 full when discovered (whew). Bath room sink still draining. Thought possibly the grey tank was overfull, but no water up in the shower.
4. Drained the grey tank fully. Water level in kitchen sink did not change. Bath sink never did back up, continued to drain until out of water in fresh tank.

Got a small plunger and used it on the kitchen sink, thinking there might be a clog somewhere, though do not know how, as it had drained properly until fresh tank nearly empty. When using plunger it was difficult to force water through drain, though some moved, apparently only the amount actually in the plunger. Sink did not drain on its own. I made sure the plunger contained no air and pumped it up and down several times. When lifting the plunger (still under water) air would come up into the plunger and some water would drain, but not all.

About the only thing I can think of is an air block somewhere in the drain line, but cannot find anything. We have never put grease or anything like that down the drain, and there are no apparent leaks.

Any ideas?