[Chrysler300] more on brakes

I decided to look more into this proportioning valve question, found out a few things. There are apparently 4-5 versions of this as it evolved, and I did not want to post any wrong stuff. So more to it than I knew.

As near as I can tell from reviewing a few service manuals in 67-78 time frame, car and truck, the earliest one on all drums was actually called a safety switch ; and the plunger between front and rear brakes , 4 lines, was spring centered (!) ; it might cut off flow to an open line too. That is on 67 dart for sure . 4 lines. The problem is that the light will come on only when/ while foot is on pedal if one side is broken. You release pedal light goes off spring centers plunger. Easy to bleed.

The 78 Dodge truck has same thing, but no springs ,and seems more definitely designed to stop up one end if excess flow. It sticks to one side light stays on without pedal. No proportioning action I can see. Tough to bleed. Seems to be drum drum or drum disc without mentioning any proportioning valve.

But I also encountered a proportioning valve which is only in the rear lines of 67 disc cars ( not the 4 line thing, there too) with disc fronts .it states it reduces pressure to drums in back to avoid early rear lockup. The test calls for putting 800 psi into it, and getting 500 out. (67 Dodge with Either Bendix of Kelsey-Hayes discs). What is in this I do not know. No adjustments . Or how it could do that, Or if proportional…or just a delaying / reducing tactic over 800psi? No other data on that setup. So some late 60’s cars do have something to slow rear drums from overdoing it when disc fronts .yet later ,78 seems not to have it

Further complicating this is the action of drum return springs with discs ? No springs on disc puck, they are going to go first on light braking (ok) but a little more pedal pressure and drums can move off springs holding them back, and then self energize ( AKA, the 300 “one front lock up” with all that drama) . All this must be carefully designed ---carefully as a system to work well together, for sure. Springs , disks, areas , piston bores, residual pressures etc.

Why dart is mentioned is I have one, fake GT 225 convert with 9” brakes, been fighting transient pull to right front for ten years. All 100% new brake parts ,every single thing, 3 times . It comes back at 3 k miles. Shoe worn unevenly across width at top, as if tilted in or out toward frame in drum. That should just center itself that way, plus held straight by “locating springs” acting like the pins we know in larger drums , as opposed to the pull off springs(in 9” drums) Starting to think bent backing plate or mismatched springs. But nothing yet. Shoes do not pull clear of drum as much as other side, especially one shoe …yet springs the same and hydraulics new , Clue is that uneven wear pattern, inner or outer part of shoe across the width of shoe hits first, at top. Noise and squeal too when this starts up . Other side is fine . 3 times. Even spindle changed en mass from 67 Cuda . Damn brake is haunted.

Fyi..

John