Kinked air hoses and/or turned angle cocks ("bottling" the air), either through a design flaw, and/or from some act of stupidity,/or negligence, and and/or up willful misconduct may, or may not have been dealt with in the design of PTC.

It really sucks, when you set the brakes and nothin' happens, though. And, if the FRED doesn't manage to dump the "bottled/trapped" air, then you're really up s#it creek, without a paddle....

A loss of braking from such circumstances, could be potentially remedied, if the FRED is properly integrated into the PTC system,, is itself, properly designed with an adequate communications protocol to be able to query and detect FRED "failure", into the "active" part of a robust PTC architecture. Then, it could be activated, due to detecting the "conditions" programmed into the final system (like "braking failure")that would be programmed into the system, as part of a comprehensive and robust PTC system "architecture" and final design.

I have not followed that part of the "architecture" and system design for PTC, in any detail. So, it might be interesting to hear from those that have some actual experience with this, or those being trained on current PTC systems, to comment on that issue?