Over the past few months, my vent selector switches to defrost and stays there. Only when I first start up the truck does it switch to what the selector is at. All other vacuum actuated things work... no problems with 4x4 or cruise. Looked over the vacuum system and cannot find any leaks.I went behind the dash and pulled off the yellow hose running to the zone selector vacuum actuator and applied a little suction to the actuator and it works fine, but does take quite a bit of vacuum pressure. Does the zone actuator become worn out or brittle?Thanks for the help, I have run out of things to try.

check here. This in on the firewall above the steering column, around the brake booster.

and here. This is right off the vacuum pump/power steering pump.

These hoses dry out and crack, letting the vacuum be reduced.

Looks like the connections are dried out but not fully cracked through yet, could be a bunch of micro leaks I guess. Anywhere sell a full connection kit?

The zone actuator is prone to failure but is a PITA to get to... Often vacune leaks are traced to the cruise control which is located under the driver's side battery. Vacume hoses are generally cheap... easiest to replace any questionable.

So I replaced all the dried out connections, still no improvement in HVAC selector. Any chance there is a few more rubber connections on the cab side of the firewall?

There could be rubber connector on the indoard side of the firewall but there's no way to get to them or the actuator with out pulling the dash back. The actuator is out on my truck... has been for some time. Defrost is the default mode. ____ Poor Design.

Actually the HVAC vacuum has a black line that runs all the way to the driver side of the trans hump in the cab there is a connector ther you might check.

Mike what is this view?

Thats the view i see when i go to the floor on the drivers side and look behind the hvac. I had to remove it when i took it out.

Mike what is this view?

Thats the view i see when i go to the floor on the drivers side and look behind the hvac. I had to remove it when i took it out.

Correct this is the driver side looking up under the dash and the driver side of the HVAC case...