Heater core

I bought a new core but the one of the tubes wasn't bent right and it would not fit in the heater box right. So I took my new core and the old one to a shop in Logotee In. he said the tanks in the old one were still in great shape and would work just fine. So he took the old tank cleaned it and put it in the new heater core and it worked fine... But I have a leak now and I'm trying to find it The fluid runs down my firewall and I get a few drops on the inside of my car on the outside of my box. I can stick my hand inside the box and feel the core and it feels dry inside. I'm hoping it's the hose on the outside.