A/c, and heat blows at defrost and floor, but not out dash vents

1993 Chrysler T+C van. The a/c and heater controls work correctly at floor setting and defrost setting, but will not work at the dash settings. Instead, if setting is changed to dash vent setting, air only blows out of floor setting. I have figured out that the A/C-heater control panel is operated by vacuum lines to control the doors that direct the air flow to the different areas. I assume I have a bad diaphragm on one of the control door chambers (correct word?), but if this is the problem, I do not know how to check to determine which chamber I need to replace. Also, can I replace the diaphragm chamber while still in the vehicle, or do I have to pull the whole heater/ac assembly out of the dash (not good) and replace it on a workbench? So, first of all, is this problem being caused by a bad diaphragm in a chamber? If so, how do I determine which chamber, and once determined, what do I need to do to replace the bad chamber?. I have located one vacuum chamber behind glove compartment and two chambers below steering column. The six air vacuum lines from dash control panel are red, brown, and yellow to chambers under steering column, black out the firewall, and light green and dark green to passenger side chamber. The brown seems to have a vacuum when set at dash setting only, and the dark green line to pass. side chamber seems to have a vacuum at all settings.