Foxx is right.
I see many brake systems that have sticky pistons.
In your service manual it recommends that you disassemble, clean, and inspect at regular intervals.
Few ever do.
What happens is you will build up goo between the two piston seals in each piston.
Then the piston tries to slide past that goo and it gets stuck.
I blow the pistons out with air, clean everything exceptionally well and pay close attention to cleaning the seals.
Your wife's tooth brush is excellent for cleaning seals.
Use a lint free rag.
Assemble with a little brake fluid.
It's amazing the difference it makes.