What is this hose?

[RRoller123]

I cut the bottom off (where it attaches to the brake booster) as it just didn't want to come off after 37 years. Slit it lengthwise, just be careful not to damage the fitting underneath. I think it was plastic if my memory serves me correctly. If it is really hard to get back on, the end can be immersed in boiling water for a short period of time to soften it. Put the new hose booster end on first, because the end at the plenum goes on much more easily, and you can get at it.