OK ok, I totally get your point now, sorry I didn't get it the first time
Yes you are right, this might be a problem.
But note that on the Sierra, wheel cylinders are not flat either, but they lean backward too, not as much as these 30°, but they do (I would say about 15°).
The other point is, when you bleed, the pistons don't move and stay in the "closed" position (if I can say so). So if in this position, the bottom of the top piston is lower than the bleeder hole (depend on the piston's thickness, I think it is but it has to be checked), the problem you describe should not occur.
Or at least that is my hope
We'll see when I bleed my brakes
