Join Date: Mar 2017
Location: Southern California
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I had similar symptoms. In full cold position the air should blow out the front vents but at times you'll notice the air can cut out under load. There is a dashpot sometimes called the power servo that sits on top of the control unit and opens the air mix door when it receives vacuum, that is the door that also allows air to flow to the front vents. It needs to receive constant vacuum to hold that door open. In my case that pot went bad so it wasn't holding enough vacuum to keep the front vents open most of the time.
Even before checking that pot, one thing to check is that you have the lines to the right ports on the vacuum reserve canister. Finnicky front vent problems can also come about by having those lines reversed. One of them should go to your Fast Idle Control Device that sits on the intake manifold and is supposed to open an extra air bypass when the compressor is on to raise the idle. The other line through the firewall is what provides vacuum for the power servo. If you're getting air out the front vents when your A/C is on but not when you're in full cold Econ mode, those lines are probably mistakenly plugged into each other's ports on the canister. A quick way to check if you have it right is put the air in Econ mode and disconnect the vacuum line to the power servo at the vacuum canister, the front vents should immediately close. That line can also get easily cracked around the end where it meets the canister and even a tiny amount of loss on that line will cause the air mix door to close sometimes - same reason it'll cut out if you're going up a hill in 5th gear and there's not enough vacuum to hold it open.
Hope this helps, I had a ton of problems with mine but finally got it all sorted out...