85 monte carlo brake problem.
 

a few years back the rear brakes on my wife's 85 ss monte carlo started locking up. so i bought new brake drums, & new brake shoes & did a brake job on the rear of the car. it did help, but not much. the only time they lock up now is when the car has sit overnight , then when we go down the driveway the rear will lock up in the gravel. after you drive it, it's good for the rest of the day. what the hell? the brakes aren't real good on the car & haven't been for a few years. i replaced the brake booster, & the proportioning valve & neither did any good. i've adjusted the rear brakes normal & loose & neither helped. there are no leaks or anything like that getting the shoes wet. i can slam on the brakes & nothing will lock up. it seems like the front brakes are a little weak, & the rear apply early. could the master cylinder be bad some how be causing this? the only two things i've not changes is the master cylinder & the metal lines, & the rubber lines on the front & rear.

So, say you go through the ritual and now the brakes are working 'right'. What happens if you do the driveway again? Will the rear brakes then NOT lock up in the gravel?

I ask, because I'd almost expect the rears to lock up before the fronts- on gravel at least- because of the lack of traction in general and the front weight transfer.

I'm wondering whether you have too much rear bias or not enough front?

If there seems to you to be a lack of front brake, the calipers might have stiff pistons that allow the rears to "out brake" the fronts. Or, like you said, the MC could be the problem. If it's original, I'd expect it to be tired.

Could be a leaky wheel cylinder that is wetting the shoes when it sits.
Wet shoes will make the wheels lock.

i've checked it after it sit for a couple of days & the shoes, drums, & wheels/tires are all dry. i've had three of these g body cars & none of them did this in the driveway. this one just started locking up after we owned it for about five years. on flat level ground with the engine idling in gear with the car rolling 2-3 mph & apply the brakes & the left rear will lock up, even just barely pushing on the pedal. it's more the left rear than the right, but they will both do it.

Have you recently rechecked the hardware (springs and retainer clips) to see that nothing's come loose or broken or is too weak?

You may want to make sure the backing plate isnt bent also.
It is odd that it only does it once a day. That is why I was thinking wet originally.