99 Chev K3500 (base) with a 6.5L Turbo Diesel - Heater Core Flush Help
 

Ok, so I got a 99 Chev K3500 (base) with a 6.5L Turbo Diesel; I found that I had no heat at all. I checked the coolant level and it was ok. No overheat issues and the temp gauge reflects normal range (185-190ish). I used the bleeder screw on the t-stat housing to try to "burp" out any air. I got warm air; but only luke warm. It wont get warmer.

I took the truck to a shop for a diagnostic and was told that I need to flush my heater core - the cost would be between $3-400 dollars.

So, my question to everyone is how do I flush my heater core? Do I have to flush the radiator as well? The block?

I would sure appreciate any help on what the steps are that are involved.

Thanks!