I missed the part where you removed/remounted your block a few times and got pretty much exactly the same results. Did you do this? Apologies if you posted doing it and I missed it.
Also (and I'm out of my depth here as I don't use vinyl tubing) are your bends sharp enough that the tubing's putting a tilting load on the block? Might the block be tilted a bit because of this?
Nice that you posted pictures, but that fade-in/fade-out is erm... irritating...
Bob
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