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The air supply and storage circuit for the brakes should be 'protected' (by a protection valve) and shouldn't be affected by the doors. This is a legal requirement, so you have a problem there.
The air reservoir that stores the service-brake air should fill (and stay filled) before any air is available to the auxiliaries such as the doors - unless someone has broken-in to the circuit and changed things. Likewise you should have footbrake air independent of the parking brake (hold-off) air - though a safety measure can be to bring on the parking brakes (which are held off by springs) when the air supply to the service brake falls below a certain value.
I'm trying to fathom what your problem is - the brakes themselves are hydraulic (though air-activated). There should be non-return valves on the storage tanks that should 'protect' the stored air and prevent it from being depleted by auxiliaries such as the doors.
Exhausting the door air shouldn't stop the service brakes from working (a legal requirement).
It seems a shame that, whilst the engine runs and can obviously supply air to operate the doors you nevertheless cannot drive the vehicle as your brakes don't work. Of course there can be other things than lack of air preventing the brakes working (such as lack of hydraulics).
You definitely need a fleet engineer to look at your vehicle and give you a diagnosis before you spend megabucks transporting it.
As I suggested, seek out local transport operators (preferably coach operators that would have used medium-sized vehicles with air/hydraulic brakes) and see if they have someone who could come and look at your vehicle in situ.
There may be panels on the floor that give access to the reservoirs and the valves that should protect the air from escaping.
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