I Have a 1998 Ford Explorer the heater works great, the A/C works great until I press on the brakes like to turn a corner then it starts blowing out heat instead of cold. If I turn off the A/C and wait a couple of minutes it will work again fine until I press the brake again. Someone told me it may have to do with the vacuum?

. Ensure all vacuum hoses are properly attached.

. Verify the brake booster check valve is working properly. Start the engine and let it idle. There should be vacuum available, when you disconnect the brake booster vacuum hose at the booster. If there's little vacuum, the check valve is not working correctly; therefore, replace the check valve and retest.

This might be an easier test for overall vacuum. Start the engine with the brake pedal depressed. If the pedal sinks slightly, the vacuum booster is working. If the pedal height does not vary, the booster or check valve is faulty. My logic is that the overall vacuum may be weak. Therefore, when you apply the brakes, there is not enough left to properly operate the blend doors.