I've never used piano wire on a pull-pull rudder. I have only used piano wire for the throttle. I have 3 planes set up with a pull-pull rudder. One uses a cable which is soldered to the clevis's, one uses standard push rods screwed on to plastic clevis's and the other one uses kevlar string which is tied in a knot to the clevis. When you look at the kevlar you think it will not hold but it is extremely strong. Your piano wire should never touch the pushrod exits. If it does then you may need to install some with larger holes. I agree with TerrellFlyer, it is a better system than push-pull.

Just my 2 pennies worth...