cleaning brass or alum with vinegar

Ok, here's the problem. I have poor heat in my truck. Over time the heater core has become less efficient at transferring heat due to whatever is going on inside the heater core, be it a buildup of rust from the engine block or whatever happens with the anti-freeze. I have used the traditional radiator flushing procedures with over the counter products such as Prestone, etc. with minor improvements. I would replace it with a new one but the repair process is very expensive if done by a shop and is a lot of work if you tackle it yourself. It involves removing the dashboard to get access. Complicating the unkown is that because I cannot see the actual heater core, which resembles a small automotive radiator, I do not know if it is alum or brass. The tubes leading in are brass but that doesn't guarantee the actual heater core is brass, it may be alum. Right now I have drained it and blown it out with compressed air. My plan is to fill it via the outlet tube with boiling white vinegar, 5% solution. Is this the right approach? Is there anything I could do to "supercharge" the solution without etching away the surfaces? Any thoughts? By the way it is a