Heater hose in line device question

[patriot10]

I have a 98 Ranger XLT, 4.0, with a/c. I'm ready to back flush the cooling system. Whenever I back flush a cooling system I use a Prestone back flush kit. On the inlet hose that runs from the thermosat housing to the firewall, about 7" away from the firewall there is a black device that looks like coolant can flow between the inlet and outlet heater hoses at that point. My question is when I cut the inlet line to intall the flushing T should the cut be before or atfter this black connecting device? I called Haynes and they didn't know what I was talking about or what this part is called. I called a local Ford Dealer and the shop foreman was clueless. I think that he just wanted me to bring the truck in and pay them $80. an hour.

[96Ranger2.3]

If i am reading correctly, That's the heater control valve. It directs coolant to the heater core, actuated by a vacuum switch. what I would do is put the t in it's place, flush engine and then reinstall the valve. Good luck.

96 ranger xlt 2wd 2.3l 5 spd