Downhill braking on a Sprinter is in the transmission.
When you let off of the throttle in 5th (hi gear) there is some but little slowing effect.
Tap the gearshift to the left and each time you will drop a gear. With each lower gear there is more braking effect from the transmission. Tap the gearshift to the right to climb up a gear. The gear you are in will show up on the dash between the tach and speedo.
The shifts are limited by RPM, if you have too many RPM for the gear the transmission will not let you shift into that gear.
Recently we were coming down Raton Pass, in our Sprinter with a toad, settled on 3rd gear for the steepest parts, little foot braking necessary.
Rick