Brake Master Cylinder problems of the 2011 Dodge Challenger

The brake pedal on this vehicle goes to the floor when braking on gravel roads. It returns functioning after pumping the pedal. The Dodge dealership in lake havasu city, az has had the vehicle twice for a week or two and last week told us that these vehicles should not be driven on dirt roads because the dust gets on the abs sensor and can cause this problem. This car has been driven on this same road for several years and has just started doing this in the last year. The Dodge dealership has not been able to duplicate the problem, so they are saying there is nothing wrong. I paid them to change the master cylinder on the first visit to the dealership because I have experienced similar problems caused by the master cylinder in the past. Of course this was before abs and cars becoming so complicated that I can no longer work on them. Changing the master cylinder did have an effect on the problem, but it did not cure it. I believe this is a potentially dangerous problem and I am concerned that Dodge has not taken this problem as seriously as they should. The first case number that Dodge gave me was 79023033. They were supposed to send a technician to the dealership to help, but I don't know if that happened. The second case number is 79309555. They said we could take it to other dealerships for a second opinion at our expense which of course will be extremely expensive. Thank you for any help you can give.