Have you tried the citrous flush method specified in the FSM? One thing that I believe is included in the FSM is that you need to degrease the system before you add the flush. That seems like a good idea for any procedure you use, in my opinion.

I can't speak for the Evapo-Rust cooling system flush, but I have used their other rust soak products and had good results.

Personally, I'd consider switching to waterless coolant after the flush, just to prolong the life of the system.