I recently got a 1970 w109 with a 3.5 engine that got idle problems and to be honest, don't know where to start.

Before picking up the car the seller and I drove to a mechanic to do safety inspection and it already had idling problems but not as bad as I'm having right now.

Before picking up the car it would actually drive and idle semi-ok , it would stall if the brake is applied. That leads me to believe that there is a vacuum leak.

Right now it doesn't idle at all, also it sounds like not all cylinders are firing up and the car barely responds to throttle.

Anyone got any suggestion what should I check?

P.S. Does air suspension somehow required for the engine to run properly? I'm asking because right now I got all the bellows removed.