I assume you have exhausted options within the automotive industry. A brass radiator would be an anomaly these days as smaller radiators are now throw-aways being aluminum and plastic. There went the old radiator shops and their hot acid baths.
Muriatic is an industrial grade Hydrochloric acid. Same stuff, Muriatic is normally limited to 20% strength. Reminds me of my industrial refrigeration days with shell and tube and shell and coil condensers and evaporative types of cooling media equipment. What ever acid you use it needs to have a copper/brass inhibitor to keep the acid from eating those metals. You did not really indicate the thickness of the deposits. The heavier the longer it takes. You will also need litmus paper to determine when the acid is neutralized by the alkaline deposits. PPE is a must, rubber gauntlet gloves and apron and face mask. Get the safety data sheet with it and follow to the tee. You add acid to the water not the other way around.
Circulation or immersion bath will be necessary to reduce acid loss and it may not be cheap if deposits are heavy. Circulation will be the fastest and easiest and use the least acid per batch. Any good industrial supply house should be able to get the acid with proper inhibitors or know where to get it.
Another milder but effective product around metals is Sulfamic Acid which is the base chemical for use in refrigeration industry cooling towers and such, less agressive that Hydrochloric. It is safer to use also. We used it when equipment could not be shut down. Dump in the sump and let it circulate with the process. Again you have to keep tabs on the PH to maintain strength.
Worst one I encountered was an evaporative condenser (lots of small copper tube bundles in and air/water shower system. It was a supermarket application for all their many compressors. It was in an area that had a huge cement manufacturing plant, there was cement dust in the air constant. Shut down for initial cleaning with acid and scrapers till the systems could gain operation. That was two days 24hr ops. We used Hydrochloric as the Sulfamic did not phase it. We then left it circulating (the unit has a large sump) during operation checking strength every 4 hours and cleaning out the sludge. That went on for 2 weeks and it got fairly clean. Our engineer designed a air-cooled condenser system which we installed the next year.
Hoe this helps and lots of luck. Above all BE SAFE acids can be very disfiguring. Any thing can be done if you throw enough time and dollars at it.
Ron
