A number of years ago I bought a 76 Dodge pickup. Okay, it was many years ago. My garden hoses were rolled up for the winter, so I went to the local car wash and used their pressure wand. I figured I had nothing to loose. I brought along a piece of plastic pipe and extra hose clamps to re-plumb if the heater core burst. After a few minutes or so of flushing, the thick green goo started looking cleaner. After a few more minutes, we were good to go. No leaks, excellent heat. Seems like that was one of the few times things worked out as planned.