I know 5hat my 73 wagon has a dual circuit master cyllender and safety valve to block off one  ircuit if there 8s a failure but am not sure if it has a true proportioning valve.

Most of the folks arund here who do larger cars insist that there is one but i have  yet to locate it.

I do know that the rear end tends to get loose during both braking and acceleration.

I just installed a rebuilt stock 2.0 with a .040 cut on the bead, stock cam advances about 7 degrees and the 2  1-1/2" SU carbs.

Just some mild mods except that it develops more low end torque.

With the larger carbs  engine starts making power about 2000 rpm and in gravel or a wet road starting out at more than about 1/4 throttle results in wheelspin.

During recent rains here i had occassion to accelerate in 2nd gear from about 25 mph and the rear broke loose and fishtailed.

Not so much on a dry road but i suspect that if I wanted i could get wheelspin as well with more throttle.

On a wet road a panic stop from 35 mph can result in rear wheel.lockup and the rear end swerving a bit and in a hard stop from 45 the car goes sideways.

On a dry road pretty much the same experience with the braking.

Any comments on the proportioning valve