If you never resolved your issue with MP09B Black Rodimus Convoy, there is a seller on eBay currently who has them available. I just ordered a set for mine after both knees came apart when I transformed him for the first time after opening him for the first time after owning him for 6 years. I was going to do the gallery for this figure when MP28 Hot Rodimus was released.
Here's the link ... http://www.ebay.com/itm/152019584803
If you are reading this later down the road, and the link no longer works, I just sent a seller a message on eBay that was selling replacement parts for the regular MP09 Rodimus Convoy.
I also have the metal knees in my pile of loot at BBTS. If the properly colored replacements don't work, I'll just do the metal knees instead.
Sorry that no one responded to your message four years ago prior to now. Hopefully we can get you back here to post again in the future!
Browning, Dai Atlas and the Big Powered Giftset, Galaxy Shuttle, Guard City, Battle Gaia, Marlboro Wheeljack, + more!