I have a 79 c6 behind a 460. What does the vacuum going from the rear of the trans up to the front of the intake?



On intake up front of carb





Back side of trans passenger side

 

That is the vacuum modulator attached to the trans to make the trans shift.

 

Yup, the modulator tells the transmission how much load the engine is under. More load=lower manifold vacuum. The modulator senses this and increases the line pressure and raises the shift point.
When the modulator goes bad, the 1-2 shift is real hard and probably won't shift automatically, you have to shift it manually and the 2-3 shift probably won't happen.