TELMA drive line retarders are really handy things to have on a skoolie. They lengthen the life of your brakes by 4X and double as a secondary brake system in the event of a failure of your main "foundation" brakes.
The TELMA units are nice and quiet also, making no noise whatsoever.
I happen to have one of these TELMA units for sale and is the perfect size for a 71 passenger bus or smaller.
Included with the TELMA unit is the relay and dashboard controls.
I have two of these units.
contact me at firstname.lastname@example.org