76 300d vacuum tank
 

I am unable so far to locate the vacuum tank in my 76 300d the diagrams show it is in the front driver fender but I am unable to find it so far, also what pressure should the vacuum pump have?

On the vacuum pump, its rebuildable by replacing the diaphragm. Ask Phil at Fastlane (forum sponsor) for the build kit. Diagnosing the pump diaphragm is easy too. Look for oil residue in the clear line running to the air filter housing. Pump pressure should be at/above 28lbs as far i know.

On the egg-crate looking air reservoir chamber, i caint help you on its exact location for the 115. But generally its the rubber boots at individual door lock mechanisms that crack and leak. Is one door lock more sluggish than the rest?

They also sell cheap rubber elbow kits. Might be good idea to replace em all as you work your way through the system. DIY degree of difficulty for vacuum lock diagnosis/repair is probly about 7 of 10 for the hassle.

Vacuum Reservoir
 

I seem to recall viewing same eons ago when I did some fender work on my 114 sedan - unless someone more knowledgeable contradicts me, here - try unbolting the plastic "shield" at the rear of your left front wheelwell, which fits up against the inside of the fender with its rubber edge/gasket. Do remove all the mud, first - and be careful - as I recall, the "gasket" was only attached with a couple of staples.
If it ain't there, it's gotta be WAY up inside the outer (knee) kickpanel, behind the covers. Before you get all that loose, however, consider a vacuum test on the line going to it, to confirm that it's leaking...
Some models apparently had two reservoirs, one in each fender...

Stan