I have a 99 Mazda B4000 4x4 with 4.0L. On the right side on top of the frame under the air cleaner box there are 2 Vaccum lines one is attached to another line line that runs back down the top of the frame in a wire run.The other is not connected to anywhere and I cannot see any place to connect it to. It is a very short lead back to the wire run it comes out of.. I am having a cylinder missfire issue and just wondering if this vac leak could be the cause. But really need to find where it goes. There is a similar line on the drivers side but only a single line. And yes 4 wheel drive works fine.
thanks.

 

I can't visualize where these are, but off-hand, if you have rough idle to stall conditions at low rpm, that's what happens with a vacuum leak. Try capping it off to see if that helps while you look for where it should be attached.
My leaks occur on the tree where the caps fall off or deteriorate.

 

In that area:

-The vac line in the electrical harness goes to the HVAC controls.

-Another goes to the Pulse Vacuum Hublock solenoid for the 4X4 hubs.

-Two plug onto to the vacuum reservoir. One is from the manifold vacuum source. The other is the vacuum out to the HVAC and Hublock solenoid.

On your 99, the reservoir is hidden from view in a recess above the plastic inner fender liner. I believe that the tee is under the air cleaner.

Be sure to check over all of the plastic hoses for damage. It is fairly common for a hose to get cut or cracked or sometimes melted by touching the RH exhaust manifold.