What is everyone's opinion of using a chemical ( prestone super flush or similar products)to help flush out a cooling system. I have a 81 pickup that has the nastiest looking coolant I have seen.

Chemical flushes are OK, just don't run the vehicle very long with it in the engine.  Coolant acts as a lubricant and has rust inhibitors.  In an old system, fresh coolant may only be good for about 12 months.

Flushes and plain water can cause leaks quickly, especially in the water pump shaft.  I'd probably only drive the vehicle with flush or plain water for about 60 miles or about 1 hour.  Remember to open your heater core.  I would not install a flush kit... just use the flush stuff.

Drain the system and flush it really well.  Replenish the system with 50/50 coolant and only use DISTILLED water if mixing.

That's just my opinion - it may be worthless.

Gnarls.