First, the locking "brake" used for parking is not known as an emergency brake no matter how convinced you are of it. It's entire purpose in a vehicle is to hold a vehicle stationary when parked. Whether or not it was effective is not the point. In rear drum brake applications, it only engages the rear shoe. Holding a vehicle still takes far less energy than it does to slow a vehicle down, Nobody with any sense would expect half of the rear drum brakes to adequately slow down a 3500 lb car especially when 60-70% of the braking is done at the front of the vehicle. It makes no sense mechanically to call the mechanism an emergency brake.
The split hydraulic sections of the master cylinders in the late 60s was a Federal mandate to provide adequate stopping pressure in the event of a fluid loss at one end or the other. Maybe your confusion is tied up with that somehow?
Also...Nowhere can the Raybestos master cylinder listed above be found for $60. The majority listed online are over $100. The lowest price I found is wal Mart at $66 and it is listed as OUT OF STOCK.
I do like the 15/16" size. I've used them in several manual disc/drum A body cars and even my Jigsaw Charger.
