1984 300TD

I replaced the key tumbler after the key was stuck in the ingnition. I followed this links instruction. http://dieselgiant.com/repairignitionlock.htm

The car will start and run for about 3 seconds then it shuts off. I’m 100% sure that this issue, has to do with the steering column lock vacuum valve. If I physically hold the vacuum valve and press in the piston to release the vacuum pressure the car will continue to run. If I release the piston then the vacuum builds and the engine shuts off

I don’t see what mechinism in the steering column / ignition presses the piston when the key is turn to the on position. Any help on this?

1984 300TD