What's that llittle round yellow thing?

[hsmith]

Greetings to all,

I have been trying to isolate a vacuum leak just about all day. On my 1984 190d (five speed) the shut off valve will work when I apply 5 Hg of vacuum - I guess that's pretty good. It stops immediately, hooked up normally it will run a while after turning it off. Anyway, coming from the dash toward the engine a solid brown & a green line with a brown stripe runs into a Y connection, then to a kind of clear line that pulls about 25 Hg vacuum - it goes on down to the engine. The green line with brown stripe hooks to a round yellow thing before it hooks to the Y. What is that yellow thing called? I think the Y connection is where the trouble is, but the yellow thing might be a problem too. When I turn her off & it keeps running, I can put my finger over the line that plugs into the yellow thing (from engine side) and she stops immediately. Sure would appreciate any help.

Thanks,

Hugh Sr.

[Chad300tdt]

On the w123s the yellow thing is the check valve for the central locking system.

[JimmyL]

On our W123's the green line goes to the climate control stuff and is probably the largest source of leaks. I'm not familiar at all with the W201's. But you should be trouble shooting from the vacuum pump {engine} to the car, not from the car to the engine.
When one of those lines into the car is unplugged and you plug where it came from and then the car shuts off then your leak is in whatever that colored line goes to.
Brown usually goes to shutoff, so I start there, with only the brown connected. Car should shut off. I then plug in one colored line at a time and see if car will still shutoff. When it won't then the leak is in that line.
Mityvac from there....

[Cervan]

the yellow thing, isolates the lock system from the other vaccume systems, that way if there is a leak in one of those systems you still have vaccume in the lock system to unlock your doors.