Chevy & GMC Vans

Most of us, myself included, have had the vents for heat/air suddenly go to the defrost position only due to a break in the vacuum line that controls it. Well, I have a similar, but different problem. Right now my vents will go from blowing normally to blowing through defrost and floor, then a little late back to blowing the setting they are on. I am guessing it is the vacuum line, but am not sure. The first time I noticed it was when driving up hills or accelerating and the vent would go to the defrost/floor. Now it seems to do it with no rhyme or reason. Is there anything other than than the vac line that might be causing this?

99% chance it's the hose. Started with a small leak that has now grown. It will eventually go to defrost only.

Thanks. I thought it might be that, but the last time it happened it went all at once.

Found it. Not too far from where I fixed it last time. Took the opportunity to replace and re-run that line completely.