Did 1969 Chevelles have some inherent issue that caused them to add this proportioning valve in 1970? Everything else in the system was the same, right?
Yes, everything was the same and I never heard of any issues with pre-70 Chevelles either. And it is to prevent the rear end from locking before
the front end (but they should both be able to lock). My guess is that it is a 'fail-safe' feature for an extreme case. Remember that we are post-"Unsafe at Any Speed
", which hit GM hard on safety issues.
I can't tell the story about the reasoning behind the prop valve, but I know the one about the metering valve that was used only on disc/drum cars (all of them). It should give you an idea of how far you have to dig to find how decisions are made.
The metering valve restricts the flow to the front brakes until a certain pressure threshold is reached (very low, around 100 psi, maybe less). The reason is to give time for the rear (drum) brakes to apply, as the shoes are retracted by springs and disc pads are not (thus react more quickly).
They are many theories as to why this is needed. None of them convinced me, because mechanically speaking, I can't see no real safety issues of having a few milliseconds response from the rear end. Until I found a theory on the web that made perfect sense to me, from someone that claimed working at GM at the time (don't ask me for a source, I lost it and can't find it again).
The problem was that when front disc brakes appeared, people weren't use to it and, like with everything new, people always have doubts about it. And it was found that some people using the cars with an automatic transmission on icy roads were applying the brakes just enough to secure the front brakes when waiting at red light for example. The result was that the rear end torque was enough to spin the wheels. The solution is just to apply the brakes a little harder, but that left an insecure feeling about this unknown «new technology» of the time for the drivers. So, just because of this very particular case (auto transmission, light braking, on icy road), all cars with front disc brakes were equipped with a metering valve.