96' Lesabre, vents switching to defrost.

I am having an intermittent problem with the air conditioning on my 96' Lesabre. When the A/C is on (max, normal or bi-level), occasionally the vents will switch to defrost mode and get stuck
there. Usually switching from heater to a/c max a few times resolves the problem (temporarily), but sometimes I have to shut the engine off to get the proper airflow again. It does not happen all the time, but usually every 3rd day or so.
I notice it usually happens when coasting, a few seconds after I remove my foot from the gas. This migh just be cooincedence, but I thought I would mention it.
I recently had to replace a bit of rubber hose for the evap emissions solenoid, it had rotted and cracked. I am thinking the same problem might exist for the vacuum feed to the vent control, but cant seem to find where it connects to the engine.
thanks
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Engine vacuum is highest when you are coasting, throttle closed.

At the least, go back and check that hose you replaced to be sure nothing else was disturbed.
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There are two lines on the evap solenoid, one runs back to the throttle body and the other goes to the can. All on the front of the engine. Nothing else was disturbed (as far as I can tell) it was a simple fix that did not take much effort.
I followed the other vacuum lines to see where they went, and could not find anything that goes through the firewall. The diagrams for the vacuum system show the feed line going through the upper passenger side of the firewall (viewed from the inside). I could not find any diagrams online for the engine compartment.
Another thing I though of was that the increased vacuum during coasting is collapsing the tube somewhere blocking the flow. I will have to check if its getting pinched somewhere possibly.
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Update, I found where the vacuum line feeds in from. Its right wherer the brake booster vacuum line hooks up. It has a plastic tube that is stuffed into the wiring harness, thats why I could not find it.
Anyway, everything there looks fine. Since the tube is plastic with a rubber transition, my collapsed tube theory is likely way off.
As I drove today, it happend again. This time the onyl way to get it working again was to shut the cr off for .5 hour or so. Then it started working ok again.
Think the controls are shot?
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could be the controls. could be!.....if it were I = remove controls, cut supply vac. line in convenient spot, 'T' in hose to attach vac. guage, hang vac. guage under dash, install controls, drive normally until it rears its ugly head again, observe vac. guage for proper vac. but then again im a stickler for diagnosis b4 replacement on everything possible......tips supply vac is purple on the old vac. driven sytems if i remember correctly..you also have a vac. cannister (plastic ball) somewhere, it has a check valve built in it........kjun
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