I used 3M Black Super Weatherstrip Adhesive to fix a tear in the drivers side rubber window gasket of my E36 coupe. It's been over a year and the repair is still going strong even with the window glass sliding over the repair on a daily basis. I think this would work well for your application after the boots are thoroughly cleaned.

https://www.amazon.com/3M-08008-Blac...N%3DB00063X38M

-Fast drying, strong, flexible, rubbery adhesive
-Withstands vibration and extreme temperature variation
-Good oil and water resistance
-Black color designed to match vehicle weather strip for virtually invisible repair