Dunno if 79 had it...sure it did. Follow the two lines from the mc down around below the fan shroud on drivers side. There are ways to reset..like the small pin facing the frame on the combo valve that you are supposed to push in and reset...this did not work for me. Also, fill mc up and take truck on road and try hitting the brakes hard in forward and reverse. That's supposed to be another way which did not work. Thrn try to open the bleeders for the front wheels, close the rears. Pump the brakes. This will in theory create pressjre that makes the valve pop free again. Did not worn for me.
I had to remove and replace mine. Cant get them at parts houses.
Here's some pics.
The reset pin is under that boot on the last pic.
Ypu should have 2 lines entering the prop valve feom the mc. In inlet for the rear and inlet for the feont. Then, two small outlets goingnto the front two wheels, and a large outlet going to the rear which splits off later down the line.
The electrical connector is to trip your red brake light when the valve is off center.
Your valve could be centered but have you bled the rears? Any air come out? Any pressure?