I have been working on a 2008 volvo vnl plow truck. Regardless if the park valve button is in or out if I make a brake application air will constantly leak out the yellow exhaust line under the cab that I believe runs to the park valve. The unit does have both Yello and red buttons but does not tow and the outlet ports of the tractor protection valve are plugged. I suspect something in the system is back feeding just do not know what. The park valve (dash valve) has been replaced and problem still exists.