1999 HVAC vent dash to floor control problem

1999 Suburban C1500, 200000 miles, with HVAC mode vent selection problem. When changing the mode selector from Dash Vent to Floor Vent and selecting heat, the air, ambient or heat never comes out of the floor vents. I saw this happen several times on this Suburban and it was intermittent over a month or so, sometimes it would work, sometimes it would not. During this time, I noticed the brakes were not very responsive, replaced front brakes and rotors and adjusted back brakes, still brakes unresponsive. Then I noticed hissing coming from the Booster after a couple of really hard engages of the brake peddle during normal driving, so I replaced the booster and for about a week, the dash vents and floor vents were working as designed when selecting the mode on the front controller, then all of a sudden, they stopped working again. Checked vacuum at booster and vacuum lines under the hood, it all looks fine. I replaced the front HVAC controller with a new one, still, front and back vents never switch to floor vents when selection is commanded at the controller. Since I dont drive this vehicle much, the battery has been fairly low a few times, and I dont always charge it before attempting to start the vehicle, so battery will indicate a low charge by slow turning crank/starter. I first noticed this problem occur after trying to start the vehicle on a weak charge. Any ideas?

You need to observe your mode door actuator operation as you rotate through the various air flow modes- does it bind, jump, go through full motion? Are all levers attached, or is there one dangling, swinging in the wind?

Mode door is behind knee bolster on driver's side, on left side of HVAC module- you need to lay on your back and bring a flashlight.

brcidd,

just so I am clear, this knee bolster is the panel directly below the steering column, correct? So , remove the trim peice that encloses the instrument cluster and audio electronics, and contains the hvac dash vents, then remove that lower panel(you are calling the knee bolster) and then follow your instructions to observe the operation of the Mode Door?

Just want to be clear on what you are calling the knee bolster.

No, do not remove anything up high, knee bolster is the panel your knees would hit in an accident, under the steering column.

There is usually no hush panel to remove on trucks (hush panel is the panel that would surround brake pedal- above your toes)- although there might be one on some models.

I usually do not remove the knee bolster panel- just wedge my head up against brake pedal (laying on my back) and shine a flashlight up to side of HVAC box to observe the electric actuator. You only have to pull upper instrument cluster trim panel loose at bottom, to remove knee bolster, if you want knee bolster out of the way.