I tried the metal post, and it didn't make a difference in bounce backs for me either. However, I did find a fix that has made the left loops shots divert to the pops at least 99% of the time. It sounds rather ridiculous though, because all it was is a piece of adhesive felt that I placed on the wire form behind Sparky, LOL. No joke. It simply keeps the ball from rolling to the left along the wireform and out the left loop after the ball hits the post. The felt is far enough away from the outside loop wall so that it doesn't interfere with left or right loop shots or super skill shots on plunges, and I've never had a ball hang up caused by it in the location I've placed it. On top of all that, you really can't see it back there when the play field is in place. So, as ridiculous as that all sounds, it works, it's cheap, and it's easily installed and fully reversible.
I'll post of picture of it when I get home (I'm at my office for the next 30 minutes), just in case any of you guys that haven't had luck with swapping posts wanna try it, and hey.. a pic of a cheap-o and easy fix like this might be good for some humor at least.
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