A/c works fine till you hit the highway above 60 it goes from the dash to the defroster. As soon as I let off the throttle it goes back to the dash.

I feel there is a vacuum leak but I checked every line I could fine and all looked well. I also sprayed with brake cleaner to listen for engine change but found nothing.

One thing I did notice is that when I shut the truck off I heard what I think is a bleed off I think from the vacuum canister. Is this normal?

I could not find any schematics online to help me locate the a/c diagram flow. Could anyone point me in that direction? I am not even sure I was hearing what I think was a vacuum canister under the hood on the passenger side which seems located in front of the heater core?

Any help would be appreciated

 

I am having a similar AC problem on my recently purchased 1997 E-250 Van, where the AC shuts off @ about 60 MPH, and resumes at lower speeds. I also suspect it is a vacuum leak, and am looking for a vacuum/air flow schematic. No luck yet, but will post it here as soon as I find one. If anyone has one, help would be appreciated!

 

I believe what you heard from the area in front of the heater core was, the equalizing of the pressures in the regrigerant system when the system is shut down.

I also believe you are on the right trac suspecting a vacuum leak. All things controlled with vacuum need to be isolated to verify the suspected area. Vehicle design is such that when a vehicle experiences a vacuum loss the the system automatically defaults to defrost as a safety feature if you happen to be driving in inclement weather.
You can disconnect some vacuum lines one at a time to isolate the leak. It would not normally be necessary to remove the MAP sensor or the brake booster lines when doing this since both are required to operate the vehicle. A c- clamp or pair of vise grips can help to pinch off some of the lines/hoses to assist with your search. Let me know how you make out. Thanks,,,,,,,,,hanky