Right, before we start (or before your mechanic starts) see:-
http://www.truckandbusforum.com/show...8&postcount=10
and:-
http://www.truckandbusforum.com/show...?t=3901&page=2
which covers wheelbrake adjustment (which should be checked first IMO).
Then, if it is decided that the master cylinder and air actuator need replacing/overhauling, here are the instructions which will assist your mechanic:-