No Park Engagement
 

Hi All:
When the transmission selector is put into park it maybe engages 1 in 10 times. This is on my 1980 450SL. If it does not engage the car can be rolled and an audiable click can be heard. Any ideas what the problem might be.
The transmission works great otherwise.
Thanks in advance for your input.

Most likely is that the small pin used to lock the transmission has been sheared off. It's a very small pin and after years of being parked without the parking brake applied, it snapped. Maybe you've got a little stub left and that's just long enough that it sometimes catches.

From the sound of it (you're able to roll the car) you're still not engaging the parking brake. You should start, and you should keep doing it after you get the transmission fixed.

I had this problem when I first purchased my 250SL a couple of years ago. It turned out to be my shifter/linkage bushings. They can be a pain to get at but certainly not as expensive as the pin described above. Hopefully, it isn't that!

Is the shifter loose when you move to drive and S or L? Shifter should "snap" into gears and be farily tight.
If so, would guess it is the gear shifter bushings....PAIN TO GET To, but should be more than about $100 to have fixed.

My low milage 560SL will roll about a foot before engaging the 'park pin' does this mean I need new bushes too?

Bushings
 

If you hear a clickety metallic noise when you move the shifter up and down the gears it is defrinetly the bushings ... don't ask me how I know.
Bad bushings will also give you a non-start condition in park, and sometimes the car will also roll when in park. Cars with this condition have been known to roll downhill (very ungood). Use the parking brake until you replace them.

Rolls a foot? That is strange....all the Mercedes I have ever had tend to roll 2-3 inches before engaging in Park....never more than that.
Do you have any play in the shifter? The selector should hold tight to the shift gate and not move.