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You will need more information from your mechanic.
The brakes are actuated by hydraulics - a 'standard' brake master cylinder is 'pushed' by an air piston (contained within the black metal unit).
The two, although physically connected are separate units.
It is possible that both are faulty - but unlikely.
Applying the footbrake sends air from the footvalve to the servo (the black thing) which pushes the hydraulic cylinder (the grey - or rusty - thing with hydraulic reservoirs containing brake fluid) which operates the wheelbrakes just like any car (well, most cars except Rolls Royces and big Citroens).
If the hydraulic master cylinder is 'faulty' then there are seal repair kits that can save you getting a new master cylinder - any mechanic should be able to overhaul this if provided with the parts.
If the air actuator is faulty, then you can get seals to repair this, too.
All this will be explained in the workshop (or training) manual if you can get one. I have one for a military 4x4 MK model which will be similar, though actual spare parts might differ.
It also tells you how to adjust the free travel between the air servo pushrod and the hydraulic master cylinder.
Ask your mechanic if he needs parts for the air servo or for the hydraulic cylinder. If he doesn't know or suggests new assemblies for both (expensive!) then he doesn't know exactly what is wrong.
You might need a second opinion to avoid costing you lots of money without solving your problem.
Are the wheelbrakes adjusted properly? There is an indicator on the air servo (the black unit) that shows how much travel there is in the system and indicates when the wheel brakes need adjusting.
Unfortunately my scanner isn't working so I cannot send you an illustration, but it says "When the red band on the indicator rod is fully exposed then the wheelbrakes need adjusting".
Adjusting the wheelbrakes is simple - the adjuster is turned until the wheel cannot be turned, then the brakes are applied with the air system fully charged, then the adjuster is backed-off until the wheel is free to rotate without binding.
It might be worth doing this before proceeding with any other possibly expensive repairs.
Of course this doesn't confirm whether the linings are contaminated or have been overheated (you need to remove the brake drums to ascertain this, but is your mechanic telling you that the indicator rod is too far out (indicating that adjustment of the wheelbrakes is needed)?
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