We backflushed a bunch of GM heater cores for no heat.The Dexcool clogged them up.Some you had to do a few times to get good heat.Never had any leaks.I had a 73 Dart that had a plugged core.Flushed it probally 10 times.Still never had good heat.Replaced the core and it roasted you out.Typically though they will be good after a good flush.That and almost never have they leaked.Rocky