flushing a heater core

I just replaced the thermostat about two months ago, along with the temperature sending unit. I brought the heatercore in the house and flushed all the rust and other debris out. It still isn't leaking.
Does it seem to flow out as fast as it goes in?? the one we had on the taurus would not put out hardly any water no matter what we tried, except I never thought about the Drano thing, did you order a new one or are you gonna try this one?? The one I have in my duster is original and as of last week it was working fine, 36 years young, of course now I jinxed myself and it will start leaking now....