Again, the search feature will give you the answer.

Do you have a Mity-Vac? Essential tool to have.

If so, have you checked the shut off valve on the IP, does it hold vacuum?

There is a yellow valve behind the oil filter adapter, should be a green vacuum line and I think a black or brown vacuum line. Green is for the lone vacuum pod on the 240's Type I heat/ac controls. The other one is for the central locking. Plug off the other one.

Again, there is extensive information available how to trace vacuum leaks on the 123 chassis, just do a search.
Alan

__________________
DO IT RIGHT THE FIRST TIME, DON'T BE A HACK
Political Correctness is NOT part of my vocabulary
and finally FIGHT CRIME...SHOOT BACK

'82 240D Stick
'85 300D Auto
Drove my first MB at age 16 1960 300SL W190