The end of the rubber bellows (Lip)on my pre facelift 91 XJS V12, keeps slipping off the round end plate. Should I apply some silicone sealant to keep it intact/sealed around it? If not for anything else, but to keep it water/airtight? I don't ever use the cruise control, but don't want the elements to ruin it.

 

Yes, seal with silicone. It's done all the time. Best method, IMHO, is to remove the servo assembly so the end plates and the lip of the bellows can be well cleaned. Be mindful of the orientation of the end plates.

Aside:

How old are your injector hoses? They have the look of originals. If so, they are wayyyyyyy past replacement date.

Cheers
DD

 

The image uploaded of the bellows assembly was from the internet. My injector hoses are fairly recent, as I had the agony of witnessing my 87 coupe go up in flames from a split hose, which is how I ended up buying my 91 V12 convertible. This car, btw...had the famous Marelli caused fire last year, 8000 damage covered by insurance.

 

I'd recommend you replace the bellows, and then silicone seal the new bellows. It's under $10 on rockauto.com.

Cheers