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Ok. I've reached a potential stopping point.
Called stoptech, they just shrugged their shoulders and said "we don't know". So, I'm on my own here.
I took the caliper apart, cleaned and rebuilt it. Actually tested it on the bench, each half separately. Both vacuum and 100psi + soapy water on the whole damn thing. I was going to even submerge it under water too.
Both halves have no leak issues. I clamped the pistons on with a metal plate to keep them from moving.
I went back to the car just to triple check - I have the brake line capped with an aluminum plug. Pedal is Rock solid.
I hook up the caliper, bleed with a powerbleed, tappign with hammer, back and forth (inner and outer bleed screws). There is no air left I'm certain. Get back in the car, pump brakes, hold, and they start sinking.
As they are sinking, I hear a "clink"... "clank" sound coming from the caliper, almost as if the immense pressure is straining the parts. But, I don't think it's related.
Matter of fact, I even clamped both halves with a C-clamp on the outside just in-case the halves were pushing apart! No change.
I'm starting to suspect this is some kind of design issue or like I mentioned in the previous post, the fluid is weeping past the piston seals just a tad bit, but not enough to leak out. Or, there is air trapped between the piston seals and the piston walls that never evacuates. I have no idea.
I'm out of options. I may have to source another set of calipers and spend $, I think it's $800/ea for new ST40s with Z4M piston sizes. And if I chance it, buy them, and same issue, then I'm going to be upset and poor.
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