Wondering what's the purpose of dual vacuum port modular valve on my 1973 f-100 C6 transmission? Needs to be replaced. Could I change it to a single port vacuum modular valve ?

 

That had to do with emissions, I forget the exact reasoning behind it though. That's the pink stripe modulator.
Yes, you can go with the single line modulator. Get the black stripe (or green stripe) and delete the vacuum line that goes in the side of the modulator.

 

I'm told that the dual-port valve was used on some EGR setups. There was a concern that the signal strength to the valve would be lowered when the EGR valve was open; so the same signal that goes to the EGR valve was ran to the secondary port of the dual-port valve to give a little extra pull. I'm also told that it's not really necessary, so there's no problem capping off the secondary port or switching to a single-port valve as Mike describes. I did the former on my Galaxie and had no problems.

 

Thank both of you very you very much..... Very informative.