There is only one line going in each door that will have an up going pressure (system blowing) to unlock the doors and will have a down going pressure (system sucking) to lock the doors.

If you hear a hissing sound in your rear door, there should be a leak. Remove the door panel and have a look (lock/unlock the car) and you will see/hear where the leak is. You do not need any smoke to find the leak.

greetingz,

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)