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Installed proportioning valve inline between the 5-way and rear brake line:
As was suggested we began by doing a "jackstand" test. The "dreaded assistant" put slight increasing pressure on the brake pedal as we turned a front and rear wheel, adjusting the prop valve to find the lightest-pressure-position where the front wheel locks and the rear wheel has braking drag but can still be moved.
Then did the road test and found the car stopped straight with no fishtailing on the panic stop.
It appears the proportioning valve, by decreasing rear braking, increases the pedal pressure required, though not by a large amount.
As was suggested we began by doing a "jackstand" test. The "dreaded assistant" put slight increasing pressure on the brake pedal as we turned a front and rear wheel, adjusting the prop valve to find the lightest-pressure-position where the front wheel locks and the rear wheel has braking drag but can still be moved.
Then did the road test and found the car stopped straight with no fishtailing on the panic stop.
It appears the proportioning valve, by decreasing rear braking, increases the pedal pressure required, though not by a large amount.