Drum brake cars don't have a proportioning valve. Originally these are for disc brake cars as these are in need of "proportioning". Are you referring to the distribution block that senses lack of pressure between front and back?
Exactly. I need to get a disc/drum proportioning valve for the front disc conversion I'm doing. Trying to determine if I can use the existing metering block with the adjustable proportioning valve and/or a new OEM style disc/drum proportioning valve.