The "second valve" is on the left hand side and is an additional block in the line running from the "front T" to the rear flex line going to the differential. It is the Proportional valve that limits pressure to 500-540 psi for the rear brakes on the Kelsey-Hayes disc brake "A" bodies. The first valve is inside the master cylinder, for the rear drum brakes only, on disc brake cars. In my first reply I was refering to the "front T" that splits the lines from the master cylinder to each of front wheels and the back wheels. There is no valve in the "front T" from 1965-?. I know the proportioning valve was moved to the "front T" by 1973. Sorry for the confusion...
