It's not the booster check valve, it's the HVAC check valve. They are not the same.

It is near the booster however. See the picture, attached.

It should be an easy fix.

 

Checked the HVAC Check valve. All is well. The other day after I'd started the truck I walked around to the passenger side and when I was at the cowl area I thought I heard a hissing. If it ever warms up in New Mexico, I'm gonna reveal that vacuum reservoir and attached hoses to see if anything is out of order there. It's really only a bother but I like things right. You wouldn't believe the time it took to find complete, functioning steering wheel controls for the radio, but I did it!