The "brake pressure differential valve" (aka shuttle valve) is housed in a brass "block". The shuttle valve reacts to pressure differences between the front braking system and the rear braking system. If the pressure differential is great enough, the shuttle valve moves (hopefully) enough to activate the warning light switch (i.e. completes the warning light's electrical circuit to ground).
The brake "proportioning valve" is housed in a cast iron "cylinder".
John
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