for something like heater core that's a PITA to get to, I would have bench tested it first before installing. Is he sure that's what's leaking? I would think it would leak INSIDE the car. Check all your hoses under hood while the engine's running with the heat on to make sure where that leak is coming from.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C