First off as you are finding out not many shops really know what they are doing. It is best to read up a little and do the job yourself. It is really not that hard to flush your coolant system. I have found that doing the flush twice is best. After you do the flush pull your radiator out and hook your garden hose up to it and back flush it and a whole lot more junk may come out. It is a pain but you will see and know your system better. That being said, in flushing the system you may have developed a pin hole in your radiator, especially if it is a slow leak. You can fix that with a stop leak that is the flake type, either copper or aluminum. You can go to WalMart or one of the chain auto supply stores and get the antifreeze you need a lot cheaper than a dealership. I get the full strength and dilute it myself.
