By placing the function control knob (18519) in the VENT position, the climate control system operates an air door within the heater air plenum chamber that directs outside air through the A/C evaporator housing, into the heater air plenum chamber and out through the registers located in the instrument panel. The volume of air discharged through the instrument panel registers can be controlled by the heater blower motor switch and temperature can be controlled through the temperature control knob.
See the orange circled sections.
Since it's not flowing enough air through dash registers, there most likely pinched or kinked Blue vacuum hose from hvac control panel to the panel air door.
Or panel air door is jambed, then the heater air plenum chamber, including these doors, must be replaced
A less likely perpetrator is a loose register air duct. This sometines happen when passengers raise their feet too high.
Here is the 1990 F Series/Bronco Pre-Delivery Shop & Electrical & Vacuum Troubleshooting Manual (EVTM), Partial by Kingfish999 in Google Drive (similar to your 91) @ 1990 Ford Truck service manuals - Google Drive
Slow scrolling for my slow Comcast Highest Cost Blast Internet..
Find page # of item desired in contents, then flip screen flip down like spinning a bearing then stopping periodically to see what page you are on.
Suggest you download the docs for faster scrolling and to use the page index.
It's probably just a vacuum leak. I've had this same problem several times over the past 15 years. When vacuum is lost to the HVAC system it defaults to the defrost and floor vents as a safety mechanism so the windshield will remain clear. My particular problem was the line coming off of the tree in the engine bay and going to the small tee/check valve and then to the blower door motor. I kept doing "quick patches" and it would last for a couple of years and then fail again (for some reason it was always on a 100 degree day in Texas). I finally replaced the whole line, including the white line from the tee to the blower motor door and I haven't had a problem since. Hopefully, your problem will be those lines because they are easier to access than the others inside the truck under the dash. Good luck!
The vent door swings very easily, i checked the diaphragm with a vacuum gun it operates all the way, the vacuum line was good. Is it possible the outside recirculating air door is causing a problem for the vents but not the feet and defrost? Thanks for the advice il try checking the lines from the tree and getting a new motor. Random question does the blower motor blow air directly infront of it and simultaneously blow air to the right of it through the evaporator core?