The foam seals in the heater box are more than likely disintegrated. New seal set are available from most Mustang supply houses. The heater box needs to be taken out though.

On the early cars, hot water is always running through the heater core. I agree with Rudi about resealing the heater box. I also agree with Walt about a shutoff to use seasonally.
Edit: Beaten by MS while tryin' ta upload the pic.

Last edited by 50vert (2/18/2022 8:13 PM)

MS wrote:

I agree with Rudi. New seals plus a hot water valve in the pressure side of the heater core hose should help. A valve in BOTH hoses is even better.

I had a valve on both hoses too....just cant remember what they were.....seems like they were for a 71 F100 with a 302.
 

Heater cores with extended tubes are available, these put the hose connections forward of the firewall.
The stock inside connections besides being messy are a major PITA to change
The issue with them is they are aluminum and like all aluminum rads and heater cores is that the antifreeze should be changed every couple of years for longevity.
I wonder if a good rad shop could add extensions to a stock copper rad.

MS wrote:

Rudi
Originals and earlier replacement cores were copper/brass.

Easy way to put the hoses on under the dash when replacing the core:

Take a 6’ long piece of 5/8” heater hose and stick each end of the hose through the firewall holes from the engine side of the firewall, leaving s loop in the engine compartment.  Pull the ends into the interior until they reach the front seat.  Attach them to the heater core before installing the heater box under the dash.

Then grab the loop of hose in the engine bay and pull on it to move the box into position under the dash.  A helper is always good, but you can pull on the hose until the four heater motor studs come through the firewall. Install the nuts loosely and then go put that long vertical screw in under the dash.  Finished!

 
That’s a great solution Steve for in cab connections.
All the ac and water hoses in my CAA  kit connect in the engine bay plus  the evap and heater core are copper.

You are very lucky to have found a mechanic you can trust to do it right.

To give you some background on this mechanic. I had a 1993 Dodge diesel serviced at his shop part of the service was to replace the rear end grease.  It had 126,000 miles on the Truck. When I picked up  the truck I made it about 5 to 10 miles from the shop and the rear end locked up. I checked and there was grease in the rear end. I called him and he had a tow truck take it back to his shop. A couple of days later he called and said come pick up the truck. There had been a problem with the ring and pinion gear. He handed me the keys. I asked what was the bill he said nothing, it came into my shop working and brokedown leaving. I wanted to at least pay for parts because just a lube would not cause the rear to fail. He said no way will I let you pay anything. That was several years ago now anything I can't do goes to his shop.