The way I heard it was DC and Cummins asked them not to. Can't really attest to the validity of that, but I heard that somewhere. With that said, since those are not available from Jacobs or Cummins or PacBrake or anyone else, I suggest the Pacbrake PRXB. That thing is trick! Sounds kinda like a mini jake. There's a definite little popping sound from it that immediately makes you think "JAKE", but beyond that, it really works! Great retarding power, even at lower rpms (1000-1500), PLUS it has the benefit of a little on-board air compressor w/ a standard connector fitting, air hose and tire chuck. Altogether a really nice piece... .