I think we want a bit more clarification here as to me it reads as if you have one cylinder of the MC working the front brakes and the other the rear brakes? Dual or split, the fail safe system should leave you with braking at front or back. As you have it now , I think, if one cylinder fails you'll only have front or back. This could mean you spinning/swapping ends if it fails under hard braking. Worse, if the front fails, the amount of braking you'd get from the rears is limited by the rear valve.
I don't think B18 Amazons got dual/split circuit front calipers in Europe (I'm open to correction here) Always rear drums of course. The single rear anti lock valve came in about '65 along with a change of rear wheel cylinder size. The 123GT used the standard ATE cast iron MC along with a servo as standard rather than as an optional extra.
If my assumption re your set up is wrong I agree with other posters about leaks causing grabbing, not helped by stuck adjusters etc. Possible the wrong size rear cylinders fitted as another possibility. The rear valves are factory set and are ususally reliable but are now "old". Swapping another one in from another type of car, ES/140/145, is risky as the settings will be different, they will be for a dual circuit anyway. You're likely to either get more or less braking rather than correct/safe braking. If you can't find a good valve you can get an adjustable one from Wilwood and start from scratch.