I have a 2002 Kia Sportage that was being driven only once a week or so for a couple of months, then it sat for a whole month without being driven. The dashboard brake light had been coming on momentarily when coming to a stop at stop lights. I didn't know what it meant at the time (but now know that was an indication that the brake fluid was getting low). When I went to drive it after it had sat for a month, the brake pedal would go to the floor and give almost no braking action. When I realized that the brake fluid was below the "min" line, I added some brake fluid to bring the level between "min" and "max", took it for a test drive, but still no improvement. I then visually inspected the brake lines from the master cylinder back to each wheel but didn't notice any obvious signs of leakage. So I bled the lines (only did the rear 2 wheels...front two wheels had bleeder screws stuck) and it turns out there was quite a bit of air in the lines. The brake pedal regained its firmness after that so I decided to take it for a test drive. The only weird thing was that when I put the car in reverse, it felt like the brakes were seized somewhere and the car wouldn't go backwards. I let off the gas, and heard a clicking sound like something released. The car then reversed properly, and I took it for a 10-minute test drive around town and everything seemed good with the brakes again.
A couple days later I go to test drive it again, and the sticking-brakes thing happened again. I put it in reverse, brakes seemed like they were locked up somewhere, let off the gas, heard a click (sounded like it came from rear passenger side wheel?), and then everything seemed normal again. I drove up the street, pulled into a parking spot, backed up with no problem this time, and returned. I decided to go for a second lap, had no problem backing up again, but after getting down the street and pulling into another parking space to back up and turn around, this time the car wouldn't go in reverse at all. It would let me creep forward (brakes dragging), but in reverse it was hard-locked-up. After trying forward-reverse about 10 times, something finally released and I was able to back up and return to my parking spot (no sign of brakes dragging or any other problems on the return drive).
This is a rear-wheel-drive car with front discs and rear drums (manual transmission). There is a possibility that the brake fluid is very slightly overfilled as I had topped it up to just above (or right at) the "max" fluid line in the master cylinder reservoir while I was bleeding the system (this was because I was moving to the right front wheel to bleed it down, but couldn't loosen that bleeder screw, so never got to bleed the front lines...since the fluid looked like it was right at the "max" line, I just left it without draining any off).
What is the most likely source of the problem? What should I check or try next?
UPDATE (4/9/2014) I pulled the rear wheels and opened up the drums to look around. There was possibly some fluid leakage inside the rear passenger side drum, but it wasn't conclusive. Unfortunately, I made a beginner's mistake and stepped on the brake pedal while the drums were off which pushed the piston out of the rear driver's side cylinder and sprayed brake fluid everywhere. So I ended up replacing both rear wheel cylinders re-bleeding the brakes (rear wheels only...front bleeder screws still stuck).
I took it for a test drive, and although it wasn't locking up in reverse, it would give a noticeable click as soon as the car started moving backwards. This time I didn't have the kids in the car so it was much quieter, and it seems to me like the clicking noise is coming from the front passenger side wheel (I did the forward-backward thing about 20 times to try to be sure). I also drove around the neighborhood with the windows down, and it seems like I'm getting a faint but noticeable scraping sound about once a wheel revolution from that same front passenger side wheel.
Any suggestions on what to try next when I pull the front wheel off? Should I take the effort to get the front bleeder screws unstuck so I can bleed the front wheels? Is it possible I have a sticking front caliper, and if so, what should I do or check for that?
UPDATE (4/28/2014) So after the 4/9/2014 update where I thought the problem had stopped, it started happening again and the brakes would be locked up when trying to back the car up. There wasn't anything wrong with the front wheel after all. The problem was in the rear passenger side drum brakes. James' answer (below) was exactly right.
I had noticed that when I parked the car, the longer it sat, the harder it would be locked up when I tried to back it up the next time. If I only let it sit for a couple of hours, I could back the car up, but it would "stick" a bit then make the loud click before moving backwards. If I let it sit for 4-5 hours, it would stick a bit harder, but would still release with a loud click when backing up. If I let it sit overnight or for a couple of days, it would be completely stuck, and would only free up if I rocked the car back and forwards a number of times. I tested by leaving it parked overnight without the hand brake engaged (manual transmission), and sure enough, in the morning nothing was stuck and I could back up with no problem.
I opened everything up again and noticed that the backing plate on the rear passenger side did have a fair amount of rust in parts, but the part I should have noticed earlier was that the three contact points where the brake shoes rest against the backing plate all had grooves etched in them that were the exact shape of the metal on the shoes. So the brake shoes were coming to rest in these little grooves. Not only that, but these grooves were rusty too, so the longer the shoes would sit stationary in those grooves, the more the corrosion would cause everything to hang up and stick together.
I couldn't get the backing plates off without a special Kia tool, so I just reconditioned them by sanding off all the corrosion with sandpaper, paying special attention to the contact points and the grooves that had been etched in them. Once I got a smooth surface again, I repainted the backing plate with high temperature auto paint, let everything thoroughly dry, and reassembled.
The car backs up perfectly smoothly now and I don't even get the clicking sound anymore, even when the car sits overnight. I'm pretty sure the clicking sound was the sound of the brake shoes "popping out" of the grooves that were etched into the backing plate by the shoes.