I want to replace my transmission vacuum line because it's pretty rusty. I wanted to know what purpose does this line serve?

 

It attaches from vacuum provided by the intake down to the rear of the transmission and attaches to the vacuum modulator. It has rubber hoses at both ends, the modulator end normally has a 90* rubber hose which is usually dry rotted and leaking as it's not easy to see and is normally never changed in 40+ yrs. which causes an unknown to find vacuum leak on many cars. If the metal line just has surface rust you could brush off the rust with some steel wool and give it a coat of paint or clear. If it's pitted then change it as it may have pin hole leaks. If there is fluid (trans) coming out of the vac. modulator it needs to be changed also.

The vacuum line connected allows the transmission to shift in synch with engine RPM.
Jon

 

Jon, that was a real good description right up to the point where you said, "The vacuum line connected allows the transmission to shift in synch with engine RPM."
The vacuum modulator controls part throttle shifts depending on the load on the engine - seperately from rpm. Under light throttle (high vacuum) you could shift into 3rd at or before you reach 20 mph while under harder acceleration it might not shift to third until 40 or more mph. The governor will shift the transmission at around 4800 rpm if the load is high enough (low vacuum) that the modulator is still not shifting.

Other than that you did a great job!
Paul

 

if u do happen to replace modulator take a pen magnet to verify the piston in modulator port strokes easily.

 

O.k. thanks for the information. However I don't see any rubber lines. I have a 65 inline 6 and it just screws into place along the lines of how your brake lines would screw into the master cylinder. I am talking about the correct part?

 

If you mean the tubes that go from the passenger side of the transmissioon to the radiator - those are the cooler lines. You can use 5/16" fuel line to replace them.

 

Sooo I got this 66 mid resto mod..... where does this vacuum c4 trans line go to ??? carb vacuum ?? manifold vacuum?? all the vacuum lines are missing... also distributor line ???

 

you hijacked -next time new post please. to answer your question man vac for the trans and carb vac (no vac at idle) for the dist