Reply to "Heater core issue...or is it working fine?"

Jimlah,
I should have been more specific. I didn't back flush the core in my cat. It was another vehicle. That being said, the principle should be the same. I do have my cat torn apart in prep for engine and trans remove. So I had to check it out. From the block a hose goes to the tube on the passenger of the tunnel. From there it moves to the drivers side and connects to the valve. It travels through the valve and core then back out on the drivers side and travels through the tunnel in the tube on that side where it connects to the water pump(suction). So I 'd say run the water back through the driver side tube. If you look under the dash you should see the heater core connections on the driver side. They do look small and I'd keep water pressure down. As Bosswrench says, they're old thin & fragile. It may be best to remove it. Hope I got it all right! Sorry if you didn't need all this detail.