Sorry if this has been answered before but I couldn't find the info anywhere. I bought a heater core backflush kit, the kind with a flush tee that goes into the return line, then started wondering if it would work on my vehicle. 1999 Suburban with front and rear climate control. There is a heater control valve before the hoses enter the firewall to front heater core. Assuming there's another one hidden back there for rear heater core. Are these valves open all the time so that I can do a heater core flush from "up top" with the truck not running? I did read somewhere that the valve only shuts off flow when controls are set to A/C. But another thing said they're only open when heat is on. Since it's vacuum controlled, and I'll be doing this flush with engine off, which is correct? I'm planning to disconnect the inlet line, clamp the hose between flush tee and water pump, and backflush the heater cores into a container, not all the way through the system. Is this possible?