I'm tracing a leak in my HVAC vacuum system. Coming off the intake manifold the vacuum line goes into a check valve that has two outputs. Where do these outputs go? One of mine was broken off and just left there, I cant find the other end of it.

This is a 1996 F150 regular cab.

 

One side of the tee after the checkvalve goes to the HVAC vacuum reservoir (tank) located on the blower case. It's a small squarish box attached to the blower case, it may be covered in insulation.

The other end leads into the cab mounted HVAC control panel. This diagram may help:

 

Well, from that I can see that it is the one to the vacuum tank that is broke. Where is that supposed to be located, under the hood?

 

Look at the photos in this thread:
https://www.ford-trucks.com/forums/1...rost-only.html

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Thats exactly what I found. The vacuum line that goes to the tank on the side of the heater box was broken.

 

Glad to see you found the root cause!

 

Bit of a twist of fate for my truck. I replaced the broken plastic vacuum line with a replacement prestone branded line, then went to work the next day - and it was better but not fixed. After the 200 mile day I got home and took another look and now the other plastic vacuum line broke. I have enough replacement line that I will change that one too and see if that fixes things. I dont know if I broke the other line when I replaced the one to the tank in the rain, or if it broke during my work day...

 

Looking at those old vinyl lines can make them break. Pony up and replace all of them. You will be glad you did.