New to me 93 Monoco Diplomat - 1st Diesel

Look at the engine exhaust - manifold then turbo then ex brake which is a flapper valve designed to close the exhaust causing the engine to become a air compressor. It works with no pedal depression and electronics that will shift the tranny to the lowest allowed gear consistent with your speed. Be aware that on a steep grade if your speed increases the tranny will up shift protecting the engine and giving you even less braking.