In '68 &'69 Mopar used a delay valve to cure your problem. It was located just behind the proportioning valve. (pic). These were installed on High Performance vehicles. In '70 Chrysler redesigned the proportioning valve so this wasn't needed. It delayed the pressure to the rear brakes so they wouldn't lock-up first.

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I love the smell of Deer guts in the morning, it smells like... VICTORY!