T3 '82 model vacuum leak in brake booster

[brujef]

Just curious when you say there is a vacuum leak? Normally that would wither affect the engine running (runs lean if the booster or vacuum lines have a big leak), or you need to apply a higher than normal pedal pressure (booster not doing it's job).
When you say it only leaks when you brake, that would be normal as there is a balance across the diaphragm until you brake. What are the symptoms though?

The boosters are tricky to get our (I assume it's under the dash like late T3's??). And unfortunately, yes, the master cylinder needs to come out also. They are not that bad to bleed, pulling the booster out is a bigger job!

Yes if it's an old original booster they can certainly get corrosion that affects them operating (not necessarily leak though). The diaphragm themselves are often good. I've re-built a few Bay window boosters and not needed to replace the diaphragm. The master cylinder bolts to the booster, so there's normally just a gasket there. (I've re-built Bay window and late T3 brakes, not an '82 but expect they would be similar).

As for tips, I'd certainly check all the vacuum lines from the engine to the booster, especially connections and the non-return valve before pulling the booster out. These are more likely to fail.

Hi Andy - thanks for your input
its an electric conversion Kombi so any issues with the infernal combustion motor and vacuum are eliminated.  i do remember the old petrol (ugh) motor running a bit rough with the brakes on, it was not an issue as the van was obtained solely for the EV conversion and with all the noise you could not hear a vacuum leak. 
there is a 12V elec vacuum pump. it runs to create vacuum and then auto switches off once adequate vacuum reached. it will not run again until the brakes are used and then it will not turn off and you can hear the vacuum sucking under the dash somewhere. once you release the pedal and the vacuum created it (pump) turns off again. also the brakes are not that effective.
this makes me think the lines are fine as there is no leak until you activate the brakes, but I will check the non return valve before going further.  is that in the engine bay or right on the booster?
first stage is to remove the dash and watch it all with the brake pedal pressed and see if the vacuum leak is obvious. 
any other input would be appreciated.
thanks, Bruce