We were having a minor issue with ours (well actually major issues... PB had to rip out original one that was leaking, and we are in the process of replacing it), but we had a unique opportunity to see it before the cap is re-poured...
Kinda hard to tell in this picture, but when it's flowing you can really see the spot that doesn't flow well, and has holes in the curtain:
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I was surprised how small, and how far back the culprit was:
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Even being able to see it, it was still tough to get out with a hacksaw blade (use carefully):
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It was just a small piece of plastic stuck in there. Here's the offender once removed (penny for size):
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After getting it out, the curtain on the sheer descent looks great again (the missing cap pieces still look horrible, but that's a different subject):
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