Ugh. Gross!.. The water I flushed out of the block after having it sit for a few days with Prestone Super Flush in it was disgusting. It looked like extremely watery diahrea. I caught the water in a couple 3 gallon buckets and it took about 3 full buckets until it started running clear.

After that, I let it sit for 10 minutes or so and backflushed again and I got another couple buckets of dirty water. I'm gonna do this a few times until I get only clear water, after filling the block with 50% vinegar or CLR. Then I'll do the same to the radiator.

FYI - I found a 3/4 FHT to 1/2 MNPT adapter at Ace hardware for $2.50 in the PVC sprinkler parts aisle. That, a 1/2 FNPT to 1/2 FNPT and a 6" sprinkler riser and I had my backflush adapter.