Is it possible to test its function? We have just checked an MGF for a potential servo leak and the way we were told to test it was to pump the pedal repeatedly with the engine off, then hold the pedal down. Turn the engine on and if it is working, the pedal should be pulled down by the servo.
However I don't know if that tests works for any servo or it is just a quirk of the MGF.
Part of the MOT includes this exact test. It's probably doing this that has generated the fail!
I don't know what to make of this at all, if you get your head under the dash of a Dolomite, any Dolomite, it will make a very slight pssshhh noise EVERY time you press the pedal with the engine running, you just have to be close enough to hear it above other noises! AFAIK it isn't just Dolomites, just mine are easier due to no parcel tray and precious little soundproofng! And it doesn't affect brake performance obviously!
If this so called fault doesn't affect brake performance, I can't see it's a legitimate reason for failure anyway, but the only REAL test is to take it out on the road and check brake down a long hill that will test if you are "running out of vacuum" which you would if there WAS a fault. This is certainly WAY beyond any MOT tester's remit! So I can't see any way round it other than a) fit another servo and see if it behaves the same way, or b) try a different tester, the problem with that, is that, these days, the second garage will have access to the first garage's fail, so will be alerted to the alleged "fault".