I assume when you say "seal" you mean the o-ring. I also suggest you replace the bleed screw. If it has been over-tightened it could be broken and won't seal, even with a new o-ring. I would also buy the brass bleed screw.
I noticed the same thing Burning Second just posted. That looks like an aluminum housing with the metal screw that comes with it (I have that on my 97). Try to tighten it a little to see if it will stop. You can put more pressure on this metal screw vs the plastic on but don't go crazy.... it's still aluminum. If not, there is a gasket, not an O ring that seals this screw. If the gasket is bad, you may have to make one.
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