Leakage in heater core at high rpm

[Djakken]

Was out yesterday and made my first of year test drive witht the Dart.
At +5 000 rpm the heater core started to leak, has never happened before. And it was a heavy leak, it spilled. Around 2 000 rpm the leak ended. Up to 5 000 rpm once agin and new leakage, down to 2 000 no leak.
Do you experts out there have any ideas why?
Am I forced to rebuild the heater assembly, or is there a possible quick way to solve the problem?

PS. I have a 273 HIPO+ in the 63 Dart

 

[TrailBeast]

Replace, or have the core repaired.
These are really the only two reasonable options.

 

[wjajr]

You need to repair or replace heater core. Quick work-a-round would be to disconnect heater hose from heater core, and loop back to water pump bypassing it.

 

[RedFish]

Loop the 2 hoses under the hood and have no heat or replace the heater core.