The deuce/half air-over-hydraulic brakes fail open (loss of air pressure leaves you with hydraulic brakes alone, loss of hydraulic leaves you with no brakes at all). However, there is a failure mode that leaves the brakes full on if the air cylinder jams.

At the time it was an acceptable trade-off; later military vehicles have more in common with trucks than tanks, I suspect.

Losing brake boost in a failure condition is the typical failure mode in most vehicles with hydraulic brakes. A hydroboost failure in modern medium trucks reduces braking performance just as much as an air pack failure in the deuce and is just as scary..

The brakes, unboosted, are supposed to still be capable of stopping the vehicle. This is also why hybrid and electric vehicles with regenerative braking, that normally use brake-by-wire with a pedal feel simulator, fail through to using the PFS as a direct unboosted hydraulic cylinder.

Losing all brakes in a hydraulic circuit failure is just because those trucks are so old they only have single circuit hydraulic brakes. Part of the A3 upgrades was changing over to a modern dual circuit hydraulic system.

So basically, the issue is that medium trucks are in a weird in-between. Not heavy enough to require full air brakes, but heavy enough that some of the safety assumptions we make about light vehicles are potentially less true.

They have a way to change tire inflation to suit the terrain. I wonder if they can change the failsafe direction of the brakes to suit the situation.