AC and heat blows through windshield ports, not lower ones?

My 2000 Astro, 75000 miles, blows AC or Heat just fine but only through the windshield ports, not the lower ones directed at the passenger/drivers, where you want it when it's hot out. Is this a mechanical issue like a blocked port, or is it the diverter switch on the dash which selects the desired location for AC/Heat?
Thanks for your input.

Check the hard nylon vacuum line connected to the LF corner of the intake manifold. This line crosses to the right side of the engine in front of the TB. It is notorious for cracking due to heat in the engine compartment.

http://www.wwdsltd.com/files/NVanHVACVacLine.jpg

That photo shows the line traversing the space in front of the throttle body, and also shows where I replaced the deteriorated nylon with rubber vacuum tubing.

The line routes to the right side and connects to a check valve here:

http://www.wwdsltd.com/files/NVanVacSourceID.jpg

awsome pics way better than words very common problem

Had same problem on my 2001 safari, vacuum line behind condenser on passenger side connects to canister under right front frame.

My 2000 Astro had the same problem, I just patched the broken line with a 2" long piece of rubber hose. Sometimes a bigger section needs to be replaced due to deterioration of the plastic line.

BTW, Defrost is the default mode on many vehicles if there is a control problem with the doors.

Finally got around to replacing the vacuum line as suggested above, and that's all it took. Thanks.

Thanks: Wife now has AC in her 1996 Astro LS , read post yesterday and forgot until today , found broken tube exactly where photo showed me , guess I won't need refrigerant after all ! sure glad I didn't start by tearing out dash or pulling engine cowling !

I am having similar problems only the vents work for sometime after starting the van, ~30min, then it shifts from the normal vents to the floor and defrost. It will not return to the normal vents until the van has been off for a day or so, then its repeats the same cycle.

Declutch : Sounds like you have another broken Vacuum line on other side of or choke side of Vac temp solenoid or switch . Best to start with basics , air filter , gas cap, PCV valve and such looking for broken or cracked plastic parts, I often stumble source of greatest mysteries wile cleaning , just remember to do one thing at a time and test after each action , and unlike old engines you should not disconnect or cap emissions for performance or convenience. Good luck .

You might also want to look at the vacuum canister (ball) on the right subframe rail just below the AC accumulator and slightly rearward of it. The plastic nipple on the ball can crack fairly easily and the ball can split at the seam after a few thousand heat/cool cycles. Heating after startup can open a cracked seam and cause loss of vacuum.

I have same problem, but need aditional help please.
2005 Astro Cargo.
So I have replaced the hose from the check valve to the Ball canister on the passenger side. Still a problem.

While trying to splice the hose from the check valve to the canister on driver side above #1 cylinder behind the AC compressor, the hose completely fell apart all the way to the boot. Removed the air cleaner housing all the way to the throttle body but can't seem to remove the boot from this canister, I don't want to break the nipple, and I just can't get my hands in there to apply enough force to pull the boot off. Also, the canister top seems to turn and 'un-click' counter clockwise, I can click it back closed. So, what is this canister, any suggestions on removing this boot with out breaking anything.
On a side note. I don't know how to remove the final right angle Throttle body cowel housing. I have the larger snorkel off but not the right angle part that is missing in the picture. I might be able to get a better grip if the boot is removable.

Thanks in advance

The piece that the nipple is attached to comes off: turn it counter clockwise until it stops and pull up. With it removed you can get at it better ;) Run a new line making sure to keep clear of hot and/or moving parts and make sure the new line fits tightly on the nipple.