Guess this mean I need a heater core

You could pump 16 lbs of pressure onto the heater core and see how long it holds.

For months my mini van had a smell of rad fluid. But no wet, no drops, no signs of a leak anywhere.

I thought the heater core had a leak and was draining out the condensation drain.

After months, I finally found some fluid and could find liquid up high on a fitting on a hose.

I replaced the part and no more leak.

I put a vacuum on the fitting and it held.
I pressurized the fitting and it held.

I put the fitting in boiling water got it hot and it didn't hold.

The PLASTIC fitting changed its characteristics with heat