rear brakes rub intermitently when backing or just after applying going forward.
I’ve installed new calypers and rotors as well as new drums and wheel cylinders on the rear. I’m wondering if the dirty brake fluid in the reservoir is causing the problem.
No, the brake fluid is not likely to be the source of that problem, but it can be the source of some other very dangerous brake problems.
Change the brake fluid a.s.a.p. unless you relish the thought of leaking hydraulic lines from internal rust and corrosion, and/or the loss of braking ability when the brakes get very hot, and/or the loss of braking ability in sub-zero weather conditions.