"The working temperature range ifor Viton® is considered to be -15 to +400 degrees F (-29 to +204 degrees C), but it will take temperatures up to 600 degrees F"

Maybe try a soldering iron which is around 800 to 900 F and see if it melts?

What is the wall thickness of your Viton tubing? I had some Viton return lines on a WVO car a while ago and the wall thickness was very thin...less than 1/16" and it pops off the nipple with very little pressure.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked