Where is the piston return spring located on your 97 4 runner?

Not sure what you are talking about. If you are referring to a part in the engine then the piston does not have a return spring. If you are referring to the piston, not quit the right term, in the brake area then in the wheels the there is a spring and in the actuator there is a spring. No other item I have ever heard of as a piston return spring. Can you elaborate?