If I have stopped the sink to hold water - then want to drain the sink - it will drain about 1/2 of the sink - then abruptly stops draining. The water will seep out very slowly over time - but not really draining. Does this have something to do with venting?

We replaced a sink - the vent was in place and working fine - as far as I know. Now we have this drain problem.

You have a partial blockage in the horizontal drain line in the wall that won't allow the sink to drain. Try a plunger if you have double sink put a towel on the other sink fill the Cloged sink half way in use the plunger if this will not help, Remove the "J" bends from the trap and send a snake up into the wall, (see image) you will hit a bend about 8 inches in but once around that you only have to put out 5 feet more. After, flush with a tub of hot water, good luck.

John