I had this issue, except for me it was FL and RR. Still opposite corners. Pretty sure that on my very first brake job, I messed up two things. 1: I just pushed the pistons back slowly in the front (and screwed in the rears) without using the bleeders to relieve the pressure. 2: I also topped off the brake fluid with some DOT4 fluid, about 20mL. This was the only non-LV fluid, added to the old stuff already in there. But I know I cleaned and lubed everything up real good.
Now, several years and one correct brake pad + rotor job later, I was having my front left and rear right wheels dragging, front left more than rear right. I could workaround by reversing, and then braking, and it would be fine for a few days. But one day, it got so hot that I started smelling it, so I decided it was time to find and fix the issue.
Attempt 1:
tear down, clean and relube all slide pins (3M silicone paste), clean and copper paste the abutments (maybe wrong word. The parts of the caliper that the pad ears touch. Let's not debate on whether or not to even do this). All pin grease was still immaculate save for one which had turned slightly grey, and the pins felt and looked fine. Actually looked better than the new-in-bag replacement pins I had on hand. No change in symptoms.
Attempt 2:
I heard that sometimes old hoses can act like one-way valves. Didn't quite explain why reversing and braking was a workaround, but nearing 100k, maybe that's too old for hoses. Replaced FL and RR hoses. Issue much worse. Every time I press brake pedal, with car off even, the FL and RR wheels lock up; both wheels release if I open either FL or RR bleeder. (Oh no, ABS is the next thing upstream!). Gravity bled several times, but no change in this new form of symptoms.
Further troubleshooting:
ABS looks expensive to replace. But to make sure that I'm not imagining things, I tested several times to make sure that both FL and RR wheels were affected the same. I locked them up and released the RR bleeder, and the FL wheel was able to rotate. I did the same thing but released the FL bleeder, and the RR wheel was able to rotate. Alas, this truely does seem like something faulty in ABS module.
I got Forscan and found no ABS module codes. Good....? Why is it acting up, then? But I did look and see what service procedures Forscan was able to surface to me from the ABS module, and found some calibrations, but also a Service Bleed function! My understanding is that this function uses the ABS pump to help you bleed brakes when you service a downstream part of the brake system. If the fix is to replace the ABS valve body, then might as well see if just sending fluid through it will jiggle something loose or something.
Attempt 3:
I attached bleeder bottle to the RR caliper and ran the ABS module service bleed procedure. It asked me what part I replaced and I told it the hose. It told me to open the bleeders and not touch the brake pedal while it's doing its thing. I obliged, then clicked OK, and the pump pumped fluid out from the RR caliper. Then I closed the bleeder and waited 45s, for whatever reason. I had filled the reservoir up to the brim, and it emptied out half of it (half of that tiny secondary reservoir; the main one attached to the MC still full).
I did the same with the FL, following the on-screen instructions.
And things are much much improved! After a long drive, the FL and RR wheels are barely warmer than the other two (by feel, no thermometer). It could even just be my imagination, that's how close in temperature they are!
Future:
Next time it's nicer out, I plan on bleeding a ton of fluid out, now that I have a nice large fresh bottle of dot4LV. I figure if dot4 is mucking up the ABS, that means it's already made its way that far, and might as well use the right fluid to flush out as much as I can from the MC and ABS pump. I don't think the calipers would care dot4 vs LV. If this was caused by damage from my first brake job, it is what it is, and I'll pay the piper later if the ABS pump goes out completely. I have forscan, so I can just get a used and reprogram it to match my car. If this was caused by that bit of DOT4, hopefully it'll be all flushed out soon. I did notice that my RR caliper is Lucas brand, so the previous owner must have had some brake issues too, but that maybe was the common seized pins issue.
