[Jack21]

My condolances. My initial thoughts would be as DV stated, What kind of pressures are you running to blow out a heater core? And what are you using for heater and radiator hoses to make the heater core the weak link in the system? If the leak is pressure induced, and if it is the heater core that is leaking. I'm thinking this is could be something vibration induced, not pressure induced. If pressure was causing this, you'd be replacing hoses by the bucketfull.

These cars do vibrate more than just a little bit, and soldered brass connections attached to a hose, particularly if it's under any stress, will come loose at the solder joint.

The original CR heater was the worst $500 abomination I've ever seen. The plans called for the hoses to go through the side of the firewall, and attach to the core where the two brass lines came out under the dash. These hoses are going to chafe and cut at the firewall, and they're going to kink. They'll have to be replaced from time to time. How is that to be accomplished without getting coolant all over the passenger side footwell? And, how does one install the new hoses through the side of the firewall once the body is installed with human sized hands? Somehow, I don't think so.

Took blower and core out of Vintage Air mini heater, butchered the CR heater box so blower and core would fit, and hose bibs came out through front of firewall. This is a separate subassembly that mounts behind the wiper motor. No fresh air intake. Car already gets plenty of fresh air. Butchered the air distribution box for heat, and defrost as a separate assembly. Only one flapper plate. Mounted this smaller version over trans tunnel. Connected the two with a flexible air duct. No vacuum controls. Manual water valve that can be opened partway with a cable in the engine compartment just off the hose bib. Cable operated flapper, in - heat, out - defrost, middle - both.

As luck would have it, defroster dries windshield quite well except for a little patch directly in front of drivers field of view. That has to be done with a towel. The heater does handle our 20 deg winters though. Toasty with top on. And 40 deg spring and fall mornings too. Toasty with the tonneau on.

If you must pull this heater out for repair, either use DV's heater, or we'll go off-line and I'll show you how I did mine.

Cutting my teeth as a truck mechanic many years ago, and a British sports car mechanic some years later, I tend to build things for ease of maintenance. CR deluxe dash is a pain the way CR has it installed. Dash now comes off with 8 screws. Electrical all unplugs. Do have to drop steering column, and that's still a pain.