74 F100 390 automatic, does anyone have a diagram showing the routing of where the vacum line are supposed to connected on the intake? My vacum modulator has two vacum lines going to it, one vacum line is connected to a intake port, the other one is connected to the same port with a "T" to connect to the top of the carbeuator which it just suck filtered air though the air cleaner, i do not hink this is correct. When the "T" is removed and both vacum line are connected to the same intake port the transmission does not operate correctly.

 

That's actually an older style dual-port modulator valve. One port goes to manifold vacuum as you've described; the other receives the same signal as the EGR valve. The EGR valve is downstream of a ported vacuum switch, which is itself sourced from ported vacuum from the carburetor. I have this same setup on my '73 Galaxie, and the signal coming from the ported vacuum switch goes to a two-port piece on the firewall. I'm not sure what this is; it doesn't look like a check valve, so perhaps it is some sort of vacuum attenuator or delay valve. I don't know for sure that it's not a check valve though.

This setup was an early part of EGR systems, as EGR had only been around for a year or two at this time. The original concern was that manifold vacuum would drop and the transmission would shift improperly when the EGR valve opened, so the extra vacuum signal was routed to the modulator valve to give it a stronger signal when the EGR valve was opened. My understanding is that this didn't prove to be entirely necessary, and you won't find this style valve in later years.

 

When you say ported vacum switch are you refering to the valves that are screwed into the water jacket chamber in the intake?

The vacum line for the EGR valve is not connected to anything on my truck from the previous owners. Based on your statement above should i just cap the second vacum line going to modulator and leave the EGR disconnected as is?