Brake hold-off valve?

[JohnZ]

The round one is the "metering" or "hold-off" valve, which denies fluid pressure to the front system until it sees 30-40 psi (to minimize "nosedive" at low speed, and to avoid behavior disconcerting to drivers while stopped on snow and ice). The one on the subframe below the driver's door is called a "pressure regulator valve" in the Chassis Service Manual, and is actually a true proportioning valve for the rear drum system.

The rectangular one directly below the master cylinder (which vendors usually call a "proportioning valve") is the distribution block, which contains the brake fluid differential pressure warning switch, which has no valving functions at all - it just illuminates the "Brake" warning light if there's a differential in fluid pressure between the front and rear systems.

In the mid-70's and later, all four functions (distribution, front hold-off, rear proportioning, and the warning light switch) were integrated into a single "combination valve".

Most vendors call them by the wrong name, and don't know what they do, either.