I'm not replacing the plug--it is the original just hanging on the wire when I found it due to some mechanics neglect.

I did try what you suggested but the problem with that is there is not enough room around the brake booster side of the plug. I can get 2/3 in but not the part behind the booster. I would just silicone the part I can't get but want it in there right like the factory did. This is a nice ride and I want to keep it that way. I'll post pics one of these days.

Failed to mention in first post--'85 300td.