A/C changes vents by itself

I have a 1992 S-10 Blazer, A/C works good except when I accelerate the air stops coming out of the dash vents, instead it comes out the defrost, now if I take my foot off the gas it comes back to the dash vents it even comes out of the floor vents when I accelerate. Any ideas what it could be?
Thanks
Joe

There is a vacuum leak in the HVAC system, or the vacuum check valve is faulty. The check valve is inline, under the hood. Follow the vacuum lines to and from the vacuum reservoir and you'll find it. It should pass vacuum in one direction only. Most auto parts stores have them in the Dorman "HELP" line and cost is under $5.

I replaced the check vavle and almost all the vacuum line, air comes out of floor vent and defroster vent, no mater were I put the lever it will not come out of dash vents. HELP it my wifes car and she is hot.
Thanks
Joe

To locate the leak, you'll need to put a hand held vacuum pump on the source line going to the mode switch in the dash. Apply vacuum and watch for leakdown. It should hold vacuum on each position of the mode switch.

old master you have been a big help in the pass do you know if I can bypass all that vacuum stuff and just make the dash vents work?
Thanks
Joe

The HVAC system uses vacuum operated servos that move doors that direct the air flow and temperature. The vacuum is supplied by the engine while it's running. Vacuum is stored in a reservoir and a check valve prevents the vacuum from flowing back to the engine. The servos are connected to the mode switch in the dash through small plastic/rubber lines. If vacuum is weak, due to a small leak, air flow and temperature become unpredictable. There is no way to manually operate the servos as they are spring loaded to provide heated air flow to the windshield in the case of a vacuum leak. Depending on the system, there are a minimum of 4 servos: Recirculate, temperature, floor/defrost, and dash vents. A vacuum leak to, or in, any one of them will cause the system to default. You need to find and repair the leak if you want the air flow and temperature to be correct.

old master, thank you again, looks like its not going to be as simple as I thought.
Thanks Again
Joe

Your symptom is exactly as mine was when I had a bad 2-way vac diaphram (servo)

I didn't replace it, but I think it was the only "2-way" diaphram under the dash.