I would look at the brakes again. You should be able to tell by looking at the rotor and the smell you perceive. The rotor will start to look tarnished even tho new (A blueish/dark tint). If you have or have access to a non-contact thermometer, compare the rear wheel rotor temps after a drive. I'm guessing the one at issue will be significantly hotter.
I admit to having not studied our braking system to any great degree to this point, however a lot of it is common sense and common operating philosophy. With that said, here are a few things I would do in the order listed:
1. Before driving it, jack the back end up so both wheels are off the ground and rotate each by hand to determine if the problem wheel takes more effort than the other one to turn.
2. Do step 1. again AFTER a test drive.
3. I can't speak to your skill level so only attempt what you feel comfortable doing. Get help from someone you trust who knows mechanics/mechanical systems and operation if possible. I would change/replace the brake fluid. The following steps are critical so follow them to the letter for your own benefit:
A. Use a kitchen baster with a piece of hose attached if you don't have access to fancy tools to suck as much brake fluid as you can out of the master cylinder. Use caution to ensure you don't drip any on painted surfaces!!
B. After you get it all out, replace it with new/fresh fluid. Fill to the top as you will be adding more anyway. Again, avoid painted surfaces.
C. Go to each wheel starting with the right rear (As it's the farthest from the master cylinder.) and draw/bleed the brake fluid out. MAKE SURE YOU KEEP THE MASTER FULL AT ALL TIMES! If you use a clear tube for bleeding you should be able to tell by the color of the fluid when you have new, clean fresh fluid coming out of the bleeder. Otherwise, use common sense based upon the amount of fluid you're putting back in the master.
ALSO: Try to AVOID pushing the pedal all the way to the floor when bleeding. Slow, steady pedal depressions are the way to go!
D. Repeat this with each wheel in order of its distance from the master, i.e. RR; LR; RF; LF. REMEMBER to keep the master full! You should go through at least a quart of new brake fluid to ensure all the old is out and you have fresh new fluid throughout the system.
After ensuring all the bleeders are closed and the master is at the correct level, pump the brakes to ensure you have a nice firm, solid pedal. Take a test drive to see if the issue has been corrected.
AGAIN, I cannot emphasize enough the IMPORTANCE of doing ONLY what you feel your comfort and skill level allow!!
I can offer a few tricks but won't for fear of you doing something beyond your comfort level. I didn't look to see where you're located or I could/would offer to help.
I think you will find there is something preventing that left rear caliper from retracting after brake application, which is causing it to remain, at least partially, applied. If you can't correct it by flushing and replacing the fluid, you have to identify and replace the actual component that is the cause.
Hope this helps!!
