Included in the possible vacuum leaks are the shut-off diaphram itself. Start by pulling the brown vacuum line off the element (shut-off diaphram back of inj pump). Put a separate piece of line to the element start the engine and suck. If the diaphram is good the motor will die. If the diaphram is bad you will get a mouth full of black nasty diesel oil. Its easy to tell which condition exists (bg).
There are so many places that can drop the vacuum so you should start here.
Bosch Master, ASE Master, L1
33 years MB technician