Water/rust/gunk etc was in the master. Got a new master & replacing the whole system.

Can you buy this new ?

Ok dokie.

since you are replacing the master are you upsizing the brakes right?  5 bolt I assume with granada rotors and the gm calipers at minimum and possibly the kit to bolt cobra brakes on the mII spindles and something to match the healthy v8's out back as well.  this little doodad is a combination balance switch (to throw the brake light when one side looses pressure) and a proportioning valve / distribution block  You can step up to a late 70's early 80's f250 master for the big bore 15/16ths or 1 inch vs 7/8ths i think and still bolt to the stock holes and put a prop valve after this or if as you say it is full of junk and you have to tinker with it anyways , chuck it all and get an aftermarket one like this
http://www.ebay.ca/itm/Black-Adjustable-Proportioning-Valve-And-Distribution-Block-/161005676342?pt=Motors_Car_Truck_Parts_Accessories&hash=item257cafa736&vxp=mtr     kills three birds with one swipe of a credit card.