You've been told correct, THEY ARE TOUGH!

I will tell you what I did on my 240D some years ago that has put off this repair. This was probably about ten years ago, and it still does not leak after I went through this process. It was a "what do I have to lose?" approach.

I disconnected the heater hoses to the heater core at the firewall. I connected two hoses in their place. These hoses were long enough to reach to a table that I had next to the fender. I connected one hose to the outlet of a pump. The inlet hose to the pump I put in a pot to suck the contents of the pot. The other hose from the heater core was also placed in the pot to return the solution. The pot was sitting over a can of sterno. I dissolved a can of "Bars Leaks" in about a gallon and a half of water and let it heat. I then turned on the pump and begin to circulate this concoction through the heater core while it was heating. I circulated it for about fifteen minutes during which time the concoction came to a boil several times at which time I would pull out the sterno for a minute or so to cool it below a boiling point for a little bit.

I then removed all this contraption and reconnected the heater hoses. While I was circulating the concoction, I had removed the radiator and block cooling system plugs and let everything drain. After the hoses were reconnected, I began filling the cooling system with 50% antifreeze.

I have drained and refilled the cooling system several times since then and the heater core remains leak free. In the course of this process a tube or two of the heater core may have been plugged up, but the heater still makes plenty of heat. It makes enough that I doubt that I would not have enough heat even if I were in Alaska.

It worked for me, I hope it may be of assistance to you.

Good luck,