2017 Berkshire Class A. Seems like it is taking longer and longer for the air pressure system to fill the air bags and for the pop off valve to pop, even at fast idle. In fact on the last start I got near max pressure and the pop off valve never opened. It took a short drive to get to the max pressure. Is there a spec on how long it should take to pressurize to max/normal pressure? Called Freightliner and they said call Cummings, called Cummings and they said it is a Freightliner issue. Any suggestions I should have the techs look at during next service visit would be appreciated.