You won't get impounded. They'll just refuse to let you cross until you have a driver with an air brake endorsement. For most of us that means getting a class "B" license, usually considered a bus license.
I checked with my DMV about an air brake endorsement on my current class "C" license. There are no subdivisions involving air brake endorsements on a class C license. The choices are either class B or class A. Class A is for truckers and there are things involved with maintaining that license, like physicals and random drug testing.