98 GMC Jimmy - No heat to floor vents.

I have a 98 Jimmy with no hot (or cool) air going to floor. Fan control, temp control, and A/C OK. Air switches between Defrost and Panel but not floor.

I have verified vacuum at control switch and checked lines connecting switch to both air control diaphrams (red/blu to lower diaphram, yel/brn to upper) and verified vacuum changing with switch. The red/blue diaphram is definately moving, and I beleive the brn/yel one is also, when switch is changed (Def-Pnl-Flr) but I do not know the correct seq./position of the diaphrams in relation to the switch positions, so I cannot verify they are moving correctly.

My only guess at this point is bad linkage or door -to- diaphram connection, but it is all hidden inside the ductwork... Any suggestions or detailed info on the diaphram positions would be appreciated.