I looked up the rear hold off valve and found that it came on "all 67-70 cars with the Kelsy Hayes 4 wheel disc brake option and high performance engines".
There are no 4 wheel disc brake cars from 67-70, unless you are talking about a different manufacturer.
A hold off valve is used in disc/drum systems to prevent pressure to the discs until a set pressure is generated int he rear system. this is to overcome the spring pressure holding hte shoes in place and overcome the very slight gap between the shoes and drum. Theoretically, this would allow the shoes and pads to apply simultaneously.
The hold off valve is not used in a 4 wheel disc system.
