I remember when going to truck driving school in 1991, there was a D.O.T. requirement for a vehicle to maintain air pressure; i.e. you applied something like 30 PSI brake pressure, and vehicle main tank pressures should drop no more than X number of PSI in one minute's time.
My new (2006) Volvo does not have a gauge for main air pressure tanks nor for applied brake pressure either!!!
ops:
Without those gauges, there is no way to verify D.O.T. compliance (if it still exists, that is).
What is up with that?