Does the car run rough?

Get a large can of WD40 or carb cleaner and spray it round all the joints on the servo with the engine running. If the tone in the engine picks up and it runs better then the joint you're spraying is leaking. Also works on any part of the induction system.

Master cylinders do not cause air leaks. If there is an air leak round the master cylinder then it's servo related. Pedal travel is a hydraulic problem within the brake circuits.