Faulty Pneumatic Door Lock System on 4KCSQ

[Doug Johnson]

Although all of the other doors on my '86 4KCSQ lock when I lock the 
driver's door with the key or by pressing the lock button on the door, none 
of the other doors un-lock when I use the key to lock the driver's door or 
when I lift the lock button to un-lock the driver's door.

Can someone please direct me to the location of the "source" of vacuum for 
the un-lock "circuit" on the doors?

I expect I've got a failure (Read: leak) somewhere before the master vacuum 
line branches to the individual doors.

Are my thoughts anywhere close to accurate???

Thanks in advance for your advice!

Regards,

~ Doug