Proportioning valve or PV, usually. I believe that on a 73 with factory disks, these are built into the metering block -- that is, the brass junction mounted below the master cylinder that the brake lines from the master cylinder route into, and that also includes the "safety switch" that activates the brake warning light. So there is no way to adjust the "proportioning", and no reason to expect it to go bad.
Earlier cars (my 67 Barracuda) had a separate, adjustable proportioning valve inline with the rear brake line. These could potentially get stuck, but there really wasn't any reason to adjust them, anyway. The "proportioning" referred to in the name isn't referring to adjustability -- it's referring to the purpose of the device, which is to slow down the rate of application of the rear drum brakes, so they do not lock up before the front disk brakes. This is needed because drum brakes are designed to have a non-linear response -- as the shoes make contact with the drum, they pull themselves tighter, increasing the amount of braking proportional to the amount of pedal effort used. Disk brakes have a linear response (more pressure equals proportionally more braking). The PV steps down the rate of pressure increase to the drum brakes.
