As far as I know the vacuum's all mechanical controlled.
Check for leaks in the hoses, or a missing hose.
Carb internals are not something I can help with, though.
That short vac hose to the depression chamber should run direct off the vaccuum in the lower carb / upper intake manifold, based on it's location. Can you blow air into the carb from this line?

edit:
When you checked the depression chamber membrane, you should have seen the secondary valve inside the carb moving as you applied suction. Did it move?

Last edited by don; 11/26/05 07:22 AM.
Don `87 Mitsu 2dr, Rubicon survivor, GModified.