I've got an issue with my dash vents.
Here's the breakdown...

Set to windshield - air blows to windshield
Set to floor and windshield - air blows to floor and windshield
Set to floor - air blows to floor
Set to floor and dash - air blows to floor and windshield
Set to dash - air blows to windshield
Set to A/C - air blows to windshield

It works sometimes works normally but most often works as described above.

I've checked for vacuum leaks using a hand held pump/gauge.
Electric vacuum pump pulls to 17in and shuts off. Gauge held steady for over 10 minutes without dropping with control in off position.
Turn the control and gauge drops about 2in for each setting and holds steady without dropping for over 5 minutes in each setting.
Vacuum pump turns back on at 12-13in and goes back to 17in.

With the glove box open, I can see 1 actuator, ( blue and yellow lines attached).
On the bottom of the HVAC box, there is 1 actuator, (white line attached)
Can't see the actuator with the red line but I also have a tee in the line to feed the heater bypass valve.

When the control is on any "dash" setting and if air is blowing to the windshield, I can move the rod on the actuator with the blue and yellow lines up against the vacuum and hold it for a second then release and the air will switch to the dash vents.
It will stay on dash vents in all dash settings after that but if I go to strictly floor or windshield settings, it won't go back to dash unless I move the rod again.

Got me stumped.

 

The air blowing to the windshield is a default set incase of vacuum pump failure or selector problems, Defrost is always what it will resort to. I have had bad vacuum chamber's go bad causing a leak. Did you do a vacuum check on the black plastic hose that runs inside from the vacuum can? Shut off the system and leave selector at dash position, pull vacuum on your tester at that black hose and make sure it holds. I have seen the check valve under the dash crack and leak as well, right where the black supply hose goes thru the firewall.

 

I didn't get a chance to work on it today.
By the time I finished the "honey do" list, the weather turned and I could hear thunder in the distance.
Barely made it in the house before the rain hit.

 

I had this same problem out on the road last year, the leak i later found turned out to be at one of the wheel fittings.
I took the vacuum elbow fitting off the 4wd soleniod/valve on the passenger wheel well, took note of which line had suction and dropped a .20 or .22 airgun pellet in.
Presto I had vent controls and now a fast way to use 2wd low since i usually keep a few pellets around.
My first thinking was that solenoid for 4wd should be fully closed if 4wd was not selected and my leak must be on the cabin side of the vacuum... I was wrong.
Valve, 4x4, and vent actuators all worked after i found the split in the hose near the wheel.

 

My truck is RWD.

 

Key off and the dash selector set to dash vents.

 

Well, I've isolated the leak...

...yes, I said leak...

to the selector/control.

It's too small to see on the gauge with the reservoir connected.
David's suggestion to test the black line with selector in dash position showed a significant drop on the gauge.
I pumped to 15" and it would drop to empty within 5 seconds or so.

I disconnected the lines at the tee to the heater core bypass valve and tested each 1.
Line to vacuum actuator tested good with no drop in vacuum for 5 min.
Line to heater bypass tested good with no drop in vacuum for 5 min.
Line to selector **** tested and vacuum dropped from 15in to 0 in less than 5 seconds.

I then applied 5psi of positive air pressure with my compressor and I can hear the leak at the ****.

 

Next question...

Can the selector be repaired/rebuilt or is it a replace it deal?

 

never heard the selector switch be the problem but good find. Report back when it's fixed. Can you take it out to see where on the switch it's leaking? I'm sure if it comes out you can find one.

 

Work has been super busy but I do plan on taking it out to verify whether it's leaking at the hose/connector at the back of the panel or if the leak is internal in the controls.