Delica Canada | Mitsubishi Van Club

I was just trying to backflush my heater core and got nowhere with it. passing water in the normal flow direction was no problem, but all I could do backwards was pressurize the line. Is there a valve somewhere behind the firewall? its pretty cramped in there and don't really want to rip it apart as I have heat, just wanted to clean it out a little.

Yes there is a valve that control temperature of the HVAC.Cold=cut the flow of coolant thru the core..Hot=coolant is flowing thru the heater core.
That valve is behind the top of the dash,over the gas pedal.