On my 83 300D, when I have driven the car and then it sits, I unlock driver's side door no problem but others won't open unless I pull up on lock. Same with trunk. If I start the car, vacuum builds up and all will lock again upon shutoff. I shut the car off and checked under the hood, I am unsure where the components are to check. I could hear some air leaking ( seemed like it was coming from large round metal cylinder behind the brake fluid reservoir) but after a minute it stopped. I then locked the driver's door and all locked fine. Could this be a vacuum pump issue or is it something in the door's themselves? And does the vacuum system have anything to do with the braking system, I read something in the Hayne's manual about it but I don't really understand it. Has anyone had a similar issue, is it easily fixed? Other than the hassle of having to reach in and unlock other doors or not being able to open the trunk after the car sits, does this pose any problem if I leave it? Thanks in advance.

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1983 300D 292,000 miles
2002 VW Jetta TDI 153,000 miles

Got Diesel?????

The sedans may have two vacuum reservoirs the store vacuum. One is the egg-shell looking thing in the trunk under the rear deck. The other may be a two- or three- sphere shaped thing inside the fender, possibly the driver's side, so hissing near the MC might mean that one. These lines are usually yellow or yellow/black. Yellow/red and yellow/green are the lock/unlock lines. They are controlled by valves in the driver's door.

But that large black metal disk behind the MC is your brake booster. The vacuum system between brakes and central locks is shared. And yes, if your booster is leaking, it may be more of problem then just the hassle of door locks.

Lock for the vacuum diagram sticky. It'll help.