I had the same problem on my RB30. It was caused by a jammed slide pin. It needed fair bit work to fix it. I had to rebuild it. Slide pin wasn't easy to pull out either. You need a minor rebuild kit, which usually includes the piston boot, slide pin boots. You also need caliper grease, which usually come in little sachets. I used valve grinding paste to polish the slide pin and the pin bore. It was a good exercise, but would've been cheaper to get a used caliper from a wrecker or fleabay. Your's doesn't sound like completely jammed, so, it might be a good idea to pull it out, clean and lube it right now.
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