So I found the culprit - it's this:
Crazy because as you can see it's meant for rubber seals, and was sold to me by racing brake (a company that is supposed to be an expert in brakes). It's actually tacky, and as soon as I dipped the Pistons in brake fluid they slid right in by hand.
I was warned not to use brake lube during this process, but obviously thought it was ok with the reference to rubber seals above. I hope others can learn from this lesson...