Possibilities are.....
A. Roll pin (not shown) thru part #11 could be sheared.
B. Adjusting bolt (not shown) thru part #8 is missing or not adjusted correctly.
C. (most likely) Brake piston, part #3 is rusted/gummed up/corroded in it's cylinder and cannot move.
D. (unlikely) Brake pucks (#1) are worn out.
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