Try tightening the clamps first, but sounds like you have a heater core leak. The solder joints where the tubes are attached are pretty fragile. I usually just cut the hose and shave down the rubber carefully until I can peel it off instead of pulling/twisting. When my '65 sprung had a leak, the new core had a crack at the joint right out of the box. Had the old one repaired at a radiator shop for $15 (30 years ago). Mine is non-AC and an easy job. Later years AC cars probably a pain...Good luck.

 

I know in a 72, It is from the inside. The fun part about a 72, is the nuts to remove it are on the firewall side, and that might mean pulling an inner fender to get to where you want. You'll have to clarify with someone with an older chevelle but I just reintalled my heater box in my el camino and all the nuts that hold on the box in the inside are on the firewall side. good luck!

Matt

 

As for that one nut that requires dropping the inner fender, the shop manual says to drill out the bolt from inside the car and then forget about it. Not exactly what you wanted to hear?