Hissing from brake servo

There is a constant hissing from the region of the brake servo on my late model TF. Well, constant when the engine is running, that is. The brakes pass the usual servo checks. And the noise isn't new - the car has passed 2 MOT tests like this. Is this normal? I wouldn't have thought so, but of course on my previous, front- engined cars the noise would have been masked by that of the engine itself.

There shouldn't be an audible hissing.
Try spraying WD40 around the servo until the hissing goes so you can isolate the problem.

Hi,   Had a similar problem. There are 2 places most likely to leak vacuum on the servo and both easy to fix.  First is joint where vacuum pipe plugs into front of vacuum cylinder, just push fit.  The second is an "0" ring in the centre where the master cylinder goes in. Just 2 bolts and a clevis pin to pull master cylinder forward. My "0" ring had been caught on assembly and had a little leak from new I think.

However if your servo has gone rusty, then time for a new one.

Cheers,  Mark

The servo  has a habit of corroding around the lower flange bolts where the master cylinder bolts to it.. I removed mine for a repaint last summer and found a tiny hole in it. 
I replaced it with one from MGFNTFBITZ which was ok, but still showed it was beginning to go in the same place. I rust treated it and repainted the servo and now its good.

Sundance

Thanks all. I shall take a closer look...

Having neither the space nor the competence to do this myself, I eventually asked a garage to replace the servo. Hissing ceased. Brakes just the same. Fuel consumption maybe improved a little.

Thanks for reporting back, it's always useful for others searching for resolutions to their problems.