Common Vac leak spots

[NoSparkNeeded]

This may be too broad a question, but are there any "common"
points of vacuum leaks in a 300TD? I need a starting point.
I replaced the check valve on the pump and all the rubber bits
in the engine compartment. I wanted to avoid taking all the door
panels off. I checked the connection to the fuel door and pretty
much everything I could without disassembly.

[uberwasser]

Taking the door panels off is what you'll need to do.

The door locks are the most common leak source, followed by the climate control.

You'll also need to test the check valves under the hood.

Here are a few guides you might find helpful. I wrote them while getting my door locks fixed up.

Disassembling Mercedes W123 Door Panel - iFixit

Mercedes W123 Vacuum Lock System Diagnosis Technique - iFixit

[NoSparkNeeded]

uberwasser that is a great write-up. Thanks for all your effort doing it.
I'll just suck it up and take all my door panels off and do it properly

[Diesel911]

You did not say what the Symptoms where.

Where the Vauucm Lines go through the Fire wall there is 3 Systems; 4 if you have Blue Tubing that goes to some sort of Seat adjustment.

Essentially you disconnect one System at a time and plug off the Vacuum Source (under the hood by the Fire Wall) end of the System. If you dissconnect and your Symptoms go away that is the System that needs further trouble shooting.

If it turns out to be the Locking System you will need to pull the Drivers Door Panel. The only thing tricky there is under the Finger Lever you pull inside of the Car to unlock the Door there is a Plastic rounded square bowel shaped Insert (black on mine) that coves up one Screw.
The front portion of the Map Pocket needs to be pushed sort of forwards and upwards at the same time.
After the Panel is off you see a Valve with 3 Connectors. You need to use your Hand Held Vacuum Tester in order to check that Valve.
The Valve on the Door needs to be tested.
If you remove the Carpet and the Plastic Threshold at the bottom of the Drivers Door you can acces the Vacuum Lines thare and you can find out which side of the Car has a Vacuum Leak from there.