Last winter, I bought a 1996 Ford F150 with a 302 engine. The previous owner had neglected the coolant, it was rusty and the heater core was clogged. I immediately did a flush and fill and back flushed the heater core. That got me some heat last winter and bought me some time.
Last month, I installed the Prestone back flush tee and Prestone flush. After repeated flushing, I ran distilled water through the engine a couple of times and felt as if I had done a good job. Refilled with Zerex G-05 (50/50). I used the Zerex concentrate and distilled water for the fill.
About two weeks ago, I noticed the coolant still had what appeared to be very fine rust particles. So, today (8/22/2016) I installed a "Dorman HELP! 55242 Fuel Line Filter" in a coolant bypass line. The truck has 128900 miles on it today. After idling until the thermostat opened, I noticed the filter turning dark. I will post back the results of running this filter in the coming months. I don't drive this truck much, so reports will be sporadic.

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You know that is essentially the ol fuel filters from the 60's and 70's.....yes they do work but, since you have no leaks, etc., and have done the right stuff (up to having the radiator boiled out).....

This is what if I may suggest......

fill the radiator with distilled vinegar and let that run through the cooling system and then sit for a few hours, flush and repeat. then drain again but place a doubled up paper towel over the funnel (going into a drain pan, etc.) which filters to less than 3 microns (much better then any of the inline filters) and you can really see where you are at.

As part of routine maintenance on my own vehicles, I......

IMHO,

1. Flush the radiator

2. When I drain the coolant, I place a doubled up paper towel over the funnelto inspect & clean/remove contaminents- in terms of contaminants, the coolant is now as clean as when it came out of the original bottle. Provided it meets all the other test I do, it is reinstalled in the vehicle.

3. I have installed a magnesium anode in all overflows (same electrolysis protection used per code for all underground tanks)- Zinc anodes are available as well (JC Whitney, etc.) specifically for automotive applications and sell for about $6- they attract all the nasty stuff that destroys radiators, pumps, hoses etc and they become the sacrificial lamb- and keeps the ph of the fluid neutral. The inside of my cooling system and related components is spotless- literally!

4. I check the ph (using a swimming pool kit) - and check the alcohol content using a conventional coolant bulb type test device ($10 at any parts store)

the results.....

1. Contaminant production is eliminated by 90%+ as a result of the neutral ph maintenance

2. Contaminants that are produced are contained outside of the system flow, in the overflow at the anode. At this point I have less contaminant in my system than if I used an inline filter because we have eliminated 90% of the production and contained the residual.

3. With the paper towel, I can actually see the residual system contaminants (which you can typically count) and by most accounts visually know if a potential problem exist by the type and color of the products. The coolant is as clean as if I had opened a fresh container.

4. The system is clean and performance level known.