Trying to figure out what this component is. On my 1989 240, there is a "Tee" fitting on the "outlet" side of the receiver/drier. There is a black plastic thimble sized cover on the top of the Tee fitting. Unscrewing the black plastic cover reveals what looks like some type of "orifice" valve. The black plastic cover has a blue o-ring under it. I'm confused as to what this thing is. I just installed a new expansion valve which is located at the evaporator. I thought A/C systems used one or the other (orifice valve or expansion valve)but not both. Any A/C techs out there that can solve this?

that is where the factory charges the ac. they are know to leak after awhile. dye up the system and when it shows up under the car, replace the hose. good luck, chuck.