The FSM gives a pretty good description. But it works front to back.

1. Check valve in engine compartment
2. Driver's side vacuum lines to master switch and lines to doors (underneath carpet)
3. Passenger side front -> passenger side rear including rear door, tank lock, trunk lock
4. Driver's side rear door.

Have golf tees (about 5) on hand to isolate lines as you work your way back.

You need to check lock and unlock lines separately. It's all a process of elimination until to come across the leaker.

MityVac best, vacuum gauge doable.

Don't waste your time pulling the door apart unless you determine that the master switch is the culprit. My '79 SD poorly maintained still had the switch intact with no apparent damage.

__________________
1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)