Originally Posted by dabac

The spring is unlikely to be the cause.
Think about it, the spring is there to disengage the brake when you release the lever. If your problem is weak braking, switching the spring won't help. If your problem had been brake dragging, then replacing the spring might help.



Well, rule-of-thumb when when working on a drivetrain with unknown service history is to always replace chain and cassette/freewheel together. It's not impossible, but the odds of new and old parts co-existing in mutual harmony are so low that they're usually not worth exploring unless there's something special about the cassette/freewheel making it difficult/expensive to replace.