Weird plumbing problem

Please don't think me insane, but when my dishwasher empties sometimes (of course not all the time) it causes a leak in the floor above it!
This has been working fine for 15 years and the only thing that has changed recently is the movement of the washing machine about 15 feet further away than it used to be in a new addition. Because of the original sill plate being there, the pitch of this waste pipe is probably a little less than optimal, but seems to drain fine with about a 1/2" pitch down to it's original connection.
In the area of where the leak shows up is the vent pipe going out to the roof, but water going up a vent pipe? and leaking besides?
There are two sink drains and a garbage disposal with the dishwasher connected to the garbage disposal, which is in turn plumbed into the same pipe as the two drains. there is no air stack connected to this set of drains, so where the pipe would normally continue up to an air stack, there is an "air gap". A small plastic valve. used for these kind of situations.
Since I was sure this was an absurd situation I filled both sinks up, pulled the plug and let them rush out - no water leak. That had to be abot 10 times the amount of water that comes out of the dishwasher during a given cycle. I would have thought it would be at a similiar or greater preasure as well, but obviously I can't recreate a motor pushing water out.
I changed the "air gap" just in case, and thought I had the problem solved, but as I was sitting doing email I heard the drip behind me - as the dishwasher was emptying on about it's 3rd cycle.
Near the vent pipe is a toilet and near that a sink. Neither cause any problem when used.
There is also another vent pipe on the other side of the house.
The dishwasher is on the first floor, it's waste water goes into the basement to go out to the septic. That run for the waste water is actually closer to the other air stack that the one that seems to have the leak.
Sorry about the long post, but wanted to include as many facts as possible.
Thanks in advance for any ideas.
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Sounds like a venting promblem your machine is putting the water in to fast for your drain to handle, is the cheater vent higher than your drain if not putt it up till it is higher than your sink drain
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nospamgoingjag wrote:

If it were a pressure/high flow problem, the dishwasher water would erupt out of the sinks.
The dishwasher probably incorporates a water heater element. Is it possible that the water is hot enough that it is creating an abnormal amount of vapor (up the vent) which is condensing and (somehow) leaking back?
See if there is a program option to have the heating element switched off. Try a load like that.
Jim
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Man, you guys are fast! Thanks for the ideas.
The vent is higher but not by much, I had extended it a few inches when we first moved in because draining the sink could sometimes cause gurgling in the shower p-traps, which stopped when I extended the air gap pipe 15 years ago, not much room to add more and be able to screw the thing on.
The heating concept is an interesting one I will have to pursue. I started to say that wasn't the problem, because I always use miser wash (no element) and air dry. But as I was getting ready to type that I remembered that my wife turned it on tonight, and hmmmm does it only do it when she uses it? maybe.
Thanks again for the thoughts, I'm going to try a run without heating followed by a run with it and see what happens - and do it at night like now when the temp has dropped suddenly.
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The difference between the sink and the dishwasher is a pump.
I'd try snaking out the drain and see what the kids dropped in it.
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