Brake System light revisited

No, they are two different functions.

The hold-off valve allows the rear brakes to apply first - the idea is that while discs apply directly, the drums must first overcome the resistance of the return springs and move the shoes out a little before any braking happens. So, the hold-off valve is intended to give the drums a "head start". Also, it's generally recognized that a vehicle stops in a more controlled fashion if the rear wheels come in first, even if it's only a little. Then, once the 'heavy braking' starts, the prop valve reduces the rear pressure to prevent the rear brakes from locking up.