I'd start at the 5-way connector that attaches to the small fitting at the vacuum pump and move down lines that won't hold vacuum. The two lines that go to the passenger side provide vacuum to EGR and ARV. The two lines that go to the driver side provide vacuum to the transmission modulator and climate control. Climate control is the trickiest to diagnose because of the number of actuators that can be routed to or isolated from the supply line. With the key out, only the defrost actuators should be routed to the supply line. Engine shutoff sources vacuum from the check valve along the big vacuum line to the brake booster. FWIW, central locking in an '87 TD has a separate electric pump

Here's a link to a 603.96 engine vacuum diagram - OM603 Vacuum & EDS Diagram . Note that some '87 300D/TDs have a split in the line at the VAC port of the transmission amplifier (item 123) that feeds a vacuum reservoir in the passenger fender... yes, the passenger fender!

Sixto
87 300D