Bmw Z4 e85 problem with break

[Aoshi]

Hi all!

I have this problem with my Z4 e85 2003, front right calliper doesn't let go the discs.
Now...I changed calliper, pads, hose and it's still the same, locks the wheel. The piston doesn't go back. But it happens only on one wheel.

I suppose I have 2 options.
Either new calliper is faulty or abs pump is.
Which is more likely?
Or...may it be something else?
Regards,