The braking system involved sounds like the one similar to what is used on the school buses on the same chassis. Braking systems have been scrutinized extensively due to failures causing injury and death. Personally I would accept nothing but a new unit complete to know it is correct and working accordingly despite the cost. The mechanic installing this unit is responsible for the working condition of that unit as is the shop that employs them. It is truly unfortunate about the failure and its untimely failure but the message seems to be the experts you consulted are being ignored and that has you irritated. Spend the money and move on with the knowledge the unit has been safely repaired for all of us. A systems analyst should know that by discounting repairs and parts you run the risk of more pre-mature component failure. Do it once, do it right and pay to have it right by the best.