Proportioning valve location

I think 1968 block on drum cars was a "pressure imbalance switch", 1967 or 1966 not. If you reduce the rear pressure (via prop valve) before the block, the imbalance switch will always be actuated.

Even when correctly plumbed, it is tricky to re-center the imbalance switch. Also, insure it isn't rusted in place (like most) so it does its job, though that is why most are "centered".