I have a 1997 F150 4.2 6cyl

so the heater stopped working, air cond. works fine
flushed the engine and core
I removed the blend door
It looks like the directional plunger is not working right, so I leave the knob on
heat, I cant eel any air or suck from the vacuum line.
the truck does not overheat.
I do have an issue with the head gasket, and I lose about a cup to a cup and
half per week of coolant/anti-freeze.
the engine temp maintains almost a constant temp of where it has always
been (summer/winter)
new cap on coolant reserve container
I cannot see where the vacuum line goes under the dash, but I know there's
a vacuum line in the engine compartment, it starts at a round disk piece
above the exit hose to the radiator, and goes to another piece on the side of
the engine with what looks like a screw cap
the truck is a 2 wheel drive nd just BASIC, I picked the STD LB for trim
because there was not a basic ick
HELP