1999 Ford Ranger A/C airflow redirects when accelerating

I suspect there is another vacuum hose off somewhere, causing the reservoir to leak down faster than usual. If all the hoses are connected, you may have a bad vacuum motor at one of the heater box valves. Find a quiet place and listen real hard under the dash, or get someone with younger ears to listen for you. After you turn the engine off you should be able to hear the leak for a few seconds. There might be a missing or bad check valve in the line between the engine and the reservoir.

Good luck