After 33+ years of opening and closing, the yellow hard plastic vacuum line for the locking system in the driver's door has cracked and doesn't hold vacuum anymore.

As for replacement, I have found at least 3 diameter options:
1.0 X 4.0 mm
1.0 X 5.0 mm
1.0 X 6.0 mm
Others?

Could someone please enlighten me as to what is the correct diameter for my vehicle?

Many thanks good M-B people-

You might consider cutting off the bad piece and slipping a rubber hose on it and connecting to the actuator in the door. If I remember right a 3/8" will fit. Or take a piece of it down to Ace Hdw.

The plastic lines on my '81 300SD are original and still perfect even though I have replaced two door actuators.

Well, I can only speak to my specific situation:
First, I have no idea where the break is. Second, most of the line is exceedingly difficult to gain access to. Meaning: After it enters the cabin, it routes through a hole in the steering column, then up and over the instrument cluster (between the dashboard and soundproofing), then down the A pillar and into the boot between the door and chassis, and then to the vacuum switch at the rear of the door. I'll be replacing the line, not troubleshooting where the leak is.

Well, I do not know the 300CD. However you might look under the carpet front passenger side for a junction box. If so, it will have all the vac connections. Just get a Mityvac and go through each one until you find the one that is leaking.

There are many threads in the forum about it. Pretty simple. Hope that is the case for the 300CD.

I will bet this turns out to be a much simpler problem than you think.

PS - what problem are you trying to solve? We do not know.

The question I posted is: "Could someone please enlighten me as to what is the correct diameter for [the hard plastic vacuum line used in] my vehicle [1985 300CD]?"

The fault: "After 33+ years of opening and closing, the yellow hard plastic vacuum line for the locking system in the driver's door has cracked and doesn't hold vacuum anymore."

If anyone knows the correct diameter size for the hard plastic vacuum line, I'd appreciate receiving the clarification.
Thanks-

4mm (outside) diameter for the lock sys plastic tubing. Less than $2/meter for OEM.

Perfect, thanks-