The residual pressure valve referenced in most of the "How to" articles on disc brakes is normally a brass spring loaded affair in one of the master cylinder ports. It is visible if the adapter fitting and brake line is removed from the master cylinder. It is not a part of all master cylinders, many do not use it. To remove it screw a sheet metal screw into it and pull it out with a pair of pliers.
...a well prepped, well driven, vehicle should do well in any terrain, including the highway.
Carburetors became obsolete during the last century... do what ever it takes to get fuel injection...It makes bigger grins off road.