I just installed a 15" round hand sink (w/o overflow) and a grid drain in a new commercial remodel. There appears to be a "back pressure" at the grid drain which makes for very sluggish drainage until the water reaches a certain height in the sink and balances the "back pressure". At this point the sink fills no further, but the water level remains high. A quick stroke of a plunger starts a siphon action that drains the sink in a flash, but once it empties, the cycle starts all over.

The plumbing is all new (6' of 1 1/2", 30' of 2", 10' of 3") to the main drain. I installed a vent pipe about 2' from the drain when I did this installation. I suspected a vent problem, but after installing an auto vent at the trap, there was no change. I even increased the trap size to 1 1/2" to no avail. I'm not a plumber, but have done many, many installations and never seen this. Any ideas?