im an mot tester, & without refering to the testers manual feel, you can only fail a brake pedal for being insecure or hitting the floor (no reserve), ie anchor/pivot points worn/loose or no brake pressure. also the master cylinder has nothing to do with air, its purely fluid operated, the servo & air tube to inlet manifold is the only brake componants that can leak air, & when one fails, the pedal will feel hard to press down with poor stopping power. if there is no hissing from the servo/pipe, & the brake perfomances are ok (over 50% of the cars wieght) the brakes should pass, ask them to show why it fails in the manual (under reasons for rejection). if no joy, try taking it elsewhere, & if it passes there, report the incedent to vosa