What is the flow rate thru a heater core? I think it’s 10-13 gpm thru radiator so I assume it’s lower thru heater hoses. I have flushed out cores using garden hose but it’s hard to do in zero deg weather.
I have a sureflo pump but it’s 1 gal/ min I figure. Not enough flow or pressure
Don’t confuse flow and pressure. Your pump could have low flow but high pressure, especially if working against a blockage.