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Having re-read your original post, I would strongly recommend that you check the indicator rod that sticks out from the 'black' unit on the sidemember (the air dual-actuator) to see whether your brakes need adjusting.
I know that you have had a mechanic look at things, but you can check yourself if the thin rod is sticking out too far (it should have a red portion) which indicates that it's time to adjust the wheelbrakes.
If the linings are too far from the drums then the hydraulics cannot push far enough (and pumping the pedal won't improve matters as it merely supplies air which pushes the hydraulics). It might be worth getting the brakes adjusted before your mechanic starts work on the servo and master cylinder.
The adjusters on each wheel are simple 'screw in' - turn until the wheel won't turn then unscrew until the wheel spins without binding. You need to jack up the axle to do this properly (and press the pedal with the engine running - or at least with air in the system - to centre the shoes when they are fully against the drum).
Once adjusted push in the thin indicator rod on the air servo and then apply the brakes - the indicator will stick out according to how much travel there is in the hydraulics.
You can adjust the wheelbrakes without jacking up the axles (for example if they are well out of adjustment) but you have to make sure that you 'back off' the adjusters so that the wheels don't bind - which will overheat the brakes - for eample if the indicator rod is along way out it's worth tuning the wheelbrake adjusters a couple of turns and checking the indicator again.
Brakes that are well out of adjustment can give the impression that the hydraulic system is faulty or that the air servo isn't working properly - exactly what your mechanic has diagnosed.
Simply fitting a new master cylinder and new seals in the air servo will not cure brakes which are well out of adjustment.
Just saying.
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