Try using some penetrating oil like PB Blaster to see if that helps, maybe get heat involved to cause the metal of the cap to expand, maybe it will expand enough that the set screw will come out. Be careful with heat that you don't melt the rubber of the valve. I have found using a couple of small drops of silicon that comes in a spray can will free up stuck or frozen aluminum to aluminum threads. If not, you can try tapping a slightly oversized torx bit in. This dust cap was stuck as the allen bolt hole had become rounded, making it impossible to remove by normal method. It seems the pigtail holder gets brittle with age and is easy to break. You need to grip the base of the screw part of the valve before trying to turn the dust cap, I had some (aftermarket) the metal if you can call it that welds itself to the valve screw, with generated heat John► R3-2009"Sparkling Black". Gone but never to be forgotten New kid on the block. Nut came off the cap, cap didn't budge.
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