It sounds like more of an issue with the drain routing since it is a new dishwasher.
I would try to route the hose with only the loop that's on the machine, in other words, as directly as possible to the sink drain.
There is a check valve on the drain but it is probably not faulty since this is a new machine. Dirty water usually signals a problem in the drain line routing.

Below is a link to the install sheet.
http://www.whirlpool.com/digita...ions%20-%20W10435040.pdf

I am not sure what you mean by a loop so I am assuming this is just an upside down U in the drain hose as shown in the install sheet.

The drain hose should be installed above the u-trap in the house plumbing.

As to a higher gas pressure this should not happen if it is installed correctly as shown in the install sheet. Above the u-trap, it is open to the sink drain so gas pressure cannot build up.

If below the u-trap then the gas build up could occur but I would have thought that you would also get an unpleasant smell (sewer gas).
A gas pressure build up would be a symptom of a problem with the plumbing stack.

Another possibility (very long shot) is that there is a partial plug in the plumbings U trap. So that when you empty the sink the water rises in the sink drain and flows into the drain hose. The loop under the cabinet should prevent this. Also you would notice that the sink drains slowly.

I looked up the parts for this unit and did not see a check valve as a separate unit but it may be built into the drain pump.

If there is extra drain hose try the following.
From the unit route the hose up to the loop under the cabinet top as direct as possible.
Then down to the house plumbing with the extra hose forming a downward U before it enters the plumbing fitting.
Be sure that the hose does not kink at the plumbing fitting.