yesterday when I tried to run the AC on MAX it stopped flowing through the front vents and switched to the windshield defroster. I put it back to normal AC and it switched to flow back through the normal front vents. Switched it back to MAX and it went back to the windshield again. I was told it was a possible Vacuum leak. But if it was a vacuum leak why would it blow AC through the vents on normal AC and then switch to the windshield when AC MAX? Any suggestions??