Here is a typical GM master with what appears to be the same size resivoirs. The front is for the front brakes. This is actually a drum/drum master, but their disc/drum were similar at first.
note that although they appear to be the same size, they are not. I just learned something too
This one is a typical distribution block with no valving. GM used the same one for disc and drum cars. This one I think is a Chrysler, but GM was very similar, save for the large weight:
note it is 5 port, with warning switch, with no button, or large nut to hold the prop spring/piston.
Here is a typical pre-71 GM prop valve used with the above valve, to reduce pressure to the rear brakes:
This one is a GM valve which holds fluid away from the front discs, allowing rear drums to catch up. It is used in the front circuit with the above:
you cant see it in the pic, but it also has a button to assist with bleeding.
Now here is a later brass combination valve for disc/drum. Note the button on the front, and the large nut on the back. Sometimes there is a nut under the button too.
and a diagram of above:
Here is a disc/disc valve. Note the lack of metering button:
GM has always used 1/4" line for the rear brakes until the axle, and 3/16" line for the front on RWD cars for as long as I can remember.
Some of their early combo valves were cast iron, and they later switched back to brass.