It's been hot so running the AC a lot. I leave it set to the top/bottom vent setting. When I accelerate the upper vents stop blowing and only comes out the bottom. This is on the freeway when I give it 50% gas or more. Any ideas?

 

Losing vacuum. Could be a problem with the check valve or the vacuum reservoir. The S/C can also cause issues with vacuum. Is there any problem with the power brakes?

Shoebox has info on location of the check valve and reservoir.

http://shbox.com/1/hvac_vacuum.jpg

http://shbox.com/1/vac_check_valve.jpg

http://shbox.com/1/vacuum_tank.jpg

http://shbox.com/1/vac_tank.jpg

 

The brakes seem ok. I don't remember the anti lock brakes ever really coming on while driving since it's a summer car. I notice sometimes they act funny slowing down over bumps/uneven pavement or like slowing down on train tracks or slippery surfaces. Feels like the anti lock feature comes on more often lately. I will check for vacuum issues. Thanks for the info.

 

It's not a matter of the ABS working or not. The power brake booster requires a stable vacuum supply. If the engine is not producing significant vacuum under load when you go to use the brakes you might have to press a lot harder on the pedal to stop the car. That would be a second indicator of low vacuum being a problem with the engine. Big cams have this problem. It seems that if your manifold was under boost, vacuum would be a problem.

But if there is no corresponding brake issue, it would point more directly to the HVAC check valve, lines, and/or reservoir.