I just fixed mine a few days ago. First and easiest place to check is the vacuum hose in the engine compartment. It runs from the passenger side right beside the firewall, there is a small three way connector with one vacuum hose running from this to the driver side where it connects to the vacuum block ( it is marked a/c, b/r, etc.). Make sure this hose is connected to the vacuum black and there are no holes in the line. This is the most common point that it leaks. Mine ended up being a vacuum hose that fell off in behind the the radio right behind the HVAC controls. Plugged it back in, and works fine now. If you recently installed a new stereo I would start here as you may have knocked off a hose.