Another "brake help" thread...

Well, blow thru the old line, both directions. There might be some rust in there. Brake lines often rust out from the inside, and usually at the lowest point, cuz that's where the water collects.
As to sticking
If you noticed the six pads where the shoes sit against, after years of service, they develop ridges that have to be ground off. for just this reason. AND
If you had water in the brakeline that rusted it as bad as I think, then your w/c's could have rust inside the bores as well. Depending how extensive that is, it could be up under the aluminum pistons, causing sticking. Extra springs, even if you could fit them in, are not the answer, just a bandaid.
New shoes, rarely properly fit the drums, new or used. This means that the initial contact patch could be just a fraction of what is should be. which means in the early stage, the rear brakes don't do much. It's gonna take a lotta miles to wear them in.
While wearing in, it's gonna take extra pedal pressure on the fronts to achieve proper deceleration. That extra pressure is simultaneously sent to the back, which usually glazes the shoes, making them LESS able to slow the drums. making you press a lil harder to make the fronts work. Thus the systrm is set up for an even longer break-in period.
Years ago when I worked as an Automotive Tech, we sent the shoes and drums down the street, to have them matched on a special grinder. This eliminated all that break-in crap.
I dunno if these machines still exist cuz so many cars are now rear disc brake.
I have 295s on the back of my car, which allowed me to eliminate the Proportioning valve which allows the back end to brake really hard. I also have a manual trans with an 11/1 360 that compression-brakes really hard. I still burn up rear pads faster than fronts.

One more source of rear brake stiction, I suppose, could be the little residual valve up at the M/C, I defeated mine. My w/c's have springs inside them which do the same thing mechanically.