Thx, All, re your input. Agree re finding one in the pic and pull. I also would prefer new ones.
I wonder if it is a residual pressure valve (10 psi) or some kind of pressure reducing valve. I am guessing, 'cuz it only has an in and an out port - that it is a residual pressure valve... to counter the brake springs.
If this is the case, then I wonder how brake balance is achieved... Would it be, say, the relative sizing of the front discs (and pistons) and the rear drums and wheel cylinders?
What I am thinking, here, is that a person could plumb-in this stuff... e.g. Willwood, or the like... and new stuff is available. It would take a few dry road (or wet road) tests.... with the proportioning valves (I would use two) set at progressively different lever-positions for ea. test...
I do wanna stay cheap... and rebuilt calipers and new rear wheel cylinders are cheap, cheap, cheap for this car...
Just wanna see if I can get it running and driving once more, more or less reliably. Sell it to a needy student... I just can't seem to decide to scrap it as it has but 145,000 km (~90,000 miles?) and it likely is good for many, many more miles. No automatic to blow up on it - and, in fact, a pleasant-shifting Mazda 5 speed... and interestingly - the best of the best set of pedals for effective heeling and toeing / double clutching of any car I've drivent!