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Do you have the valve from the same truck as the rest of the components? That is likely to be a good start. The square body trucks had a slightly better weight distribution but they were still pickups and they still had to prevent rear lockup.

Even better might be a height adjustable valve from some of the larger pickups. That way you can dial it in to reduce rear braking when empty and if you ever load weight in the rear you'd get additional braking automatically.