So a little back story, I bought a '86 2.8 4WD S-10 Blazer with Tahoe trim. The truck had a remanufactured engine put in about 4 years ago and sat for the last 2 years. I have done a tune up and fixed a lot of the small issues but this one has been bugging me.

When I accelerate my vents go to defrost mode. After researching this common problem I have found it is likely a bad check valve or a leaking hose. I have traced the hoses and replaced as necessary but the problem still exists.

I started looking for the check valve but I cannot find one anywhere. I have drawn a diagram showing the lines I traced. Is there a chance the check valve went missing during the engine swap? Is there a chance my truck didn't have a check valve? Should I install a check valve and if so what type? I can't seem to find a direct replacement on autozone/advanced.

Thanks for any help and clues.

 

There's a large vacuum line that starts at the intake manifold (front left of throttle body) That should go to the check valve, then split out to appropriate destinations.

https://www.google.com/search?q=vacu...x-a&channel=sb