Late 69 distribution block after proportioning valve HELP

[fy469rtse]

On a late 69 disc brake the secondary proportion block after distribution block,
My question is what's the pin for in this valve , it has a small rubber boot , this is proportioning valve for front only
Examining all options to brakes,
Bleeding them and can't seem to get any fluid to rears , have used compressor type to vacuum system, front come up great , rears no luck , should I suspect distribution block is bad and replace , everything to brakes brand new , quality is questionable , ?
Bench bleed master before install , new lines , new everything, theres not a part that's not been replaced , so should I suspect brand parts are no good ?
Need advice , not sure what to do next  ? Very frustrated , after many issues with brand new stock parts

[terrible one]

This should help: https://www.musclecarresearch.com/1970-71-mopar-metering-valve-autopsy

[fy469rtse]

Thanks for the help 

[fy469rtse]

Still going with these brakes ,
Found slight seeping at the rear of master cylinder, drained fluid and remove , pulled it open and crap inside new part ? , forward plunger stuck  got it out after awhile, cleaned up the shaft , seals in good shape, put it back together, bench bleed master and installed it in the car , fitted all the lines
and vac brake bleed again, fronts straight up , rears still can't get fluid after many hours of pumping , vac bleeding , ok time to see where fluid is getting to, pumped up brakes and cracked open a couple of connections , no fluid , fluid gets to proportion valve and no further,
Valve is junk and not too confident on the master cylinder now ,
Time to buy both brand new and wait for them to arrive and take two on brakes, will update on this once new parts arrive , mind you these were new to start with ?