i think i found the answers i was looking for
About what set-up you can have:
http://www.chevelles.com/forums/show...00&postcount=2
The metering valve is the valve on the front part of the previous picture. It opens only at a certain pressure threshold (around 100 psi IIRC). This gives times for the rear to build up pressure to compensate for the rear shoe springs such that front & rear brakes are applied at the same time.
The prop valve is the screw-in valve on the rear part of the previous picture. It some sort of pressure reducer. (more info
here)
The distribution block does only as its name says: It distributes the fluid to the front & rear brakes. It also have a switch (in the middle part) that lights up the brake warning light if the front or rear brakes show a pressure lost (i.e. a leak):