Why is the vent pipe from dish washer over flowing ?

. In the top of an air gap is an insert with two holes. Inside the air gap are two divisions. One division connects to the inflow side of the air gap (from the dishwasher) and the other connects to the outflow side (to the disposal). If the top insert is put in wrong, water will flow from it when the dishwasher empties. For correct operation of the air gap both holes in the insert need to be positioned over the outflow division of the air gap.


In case you ever want to do away with that pesky counter top air gap and install a high loop Air gap.(see image) I'll be more then happy to help.
Note: In all the hundreds of high loop air gaps that my company's installed we have yet to get our first complaint or service call. This holds true in the high loops that we have advised installing on The Plumbing Page right here.

Good luck, Tom

... also, it could be that the 7/8" discharge hose has food particles in it preventing good flow to the garbage disposer. Furthermore, make sure that hose is not kinked or that it doesn't have sharp bend in it.