'79 240D Won't downshift to 1st gear
 

I purchased my first MB 3 days ago, a '79 240D automatic transmission with 240,000 miles. I know it has a vacuum problem because the door locks are slow and the engine doesn't shut off until about 30 seconds after I turn the key. Also, I noticed it doesn't downshift to first gear unless I move the shift to the "L" position. Is it likely this is due to the vacuum problem? What else could cause this?