first of all, you started at the wrong point. place to start testing for leaks is drivers side corner in engine compartment near the hood spring. right there is the vacuum line splitter and check valves where part goes to the door/trunk vacuum line and part goes to the heater/ac system. then work your way to the 4-branch connector.
from memory, I don't think all 4 of the lines have it but i recall that 2 lines have inserts visible when you pull off the vacuum hoses. I'll explain those inserts: if you look at the vacuum diagram there are 2 loops that don't make sense. within those loops are these inserts, basically they stop about 90% of air flow. thier purpose is to slowly release vacuum after engine shut-off on the re-circulate vents. I assume the purpose is to keep outside air out when the car is not in use. these inserts are notated by thin brass rings on the outside of the black vacuum tubing which points out their location. if you pull them out they look like cigarette filters.
checking for vacuum leaks is not difficult but should be done methodically. somewhere I have a copy of Mitchell's guide on how to do this. I found it much simpler to follow than the MB cd manual. Mitchell's guide is available at my public library reference section. check yours. if you don't find it I can likely find my copy and send it to you.