start problem / vacuum

I am not mechanical and will appreciate any help with this problem on my 1989 300 gd. The vacuum switch that cuts the fuel supply to the engine switches off normally however the switch will only realese to allow a restart after turning the key three or more times, I can then hear the vacuum release. If I then do not start immediately the vacuum will sometimes pull the switch closed again.
What I have done so far is check the vacuum switch which has v strong return spring and appears o.k. I have swapped the ignition switch/valve etc. but have exactly the same problem. I suppose I could have two ignition switch mechs. with identical problems but this seems unlikely. When the vacuum switch is in the off position (vacuum pulling) I have pulled the brown vacuum pipe off the switch and it springs up. The same thing happens when I pull the brown vaccum pipe off the T connection to the larger white pipe at the front of the engine. when I substitute my finger for the brown pipe I can feel the vacuum pulling, even with the engine not running (this T connector also appears to be a valve). I guess all this leads me to conclude that the vacuum is pulling all the time (is this right) and or it cannot be released correctly. The vacuum cut off switch and ignition mech. appear o.k....... what next please? I'm lost.