Vacuum Problem?

[99strovan]

I have a 99 Astro. Under normal conditions the blower fan works just fine blowing air out the front vents. However, when I go up a hill or the van shifts down for power the air shifts to blowing out the defrost vents. I have checked vacuum lines. They appear to be okay. When I checked the vacuum tank, a few drops of water came out. Any ideas?

[Blue Bowtie]

Take a closer look at the vacuum T at the intake manifold and the hard plastic lines. A split or small leak will create the symptoms you describe. There is also a vacuum check valve in the line near the right side, and a failure of that will create the same conditions.

[racer504]

Just wondering if any of the suggestions by Blue Bowtie helped your problem? I have exactly the same problem and it seems to be getting worse. I replaced all the lines and check valve, and made sure the tee was OK. I also replaced the PCV valve. I'm beginning to think something else on the intake is leaking but not sure what to look for.

[mike7051]

i know this is old and you guys have probably figured it out but there is a vacuum line that runs from the rear of the intake driver side to the heater control firewall area pass side this line becomes cracked around the front of the intake by the thermostat housing. when its cracked ac and heat work fine but under accel you lose your vents or it will seem as if the ac or heat has stopped