SOLVED!!!
Hi folks, I posted in this thread a few weeks ago, working with Igofar to solve my rear brake dragging issue and overheating disk. After much testing, thinking and discussing, multiple dismantles, inspections and remounts of the brake assembly, I concluded my issue could only be mechanical (not hydraulic related) as the pads stayed squeezed when the fluid pressure was manually released. It made no difference if the braking pressure was applied using only the rear brake pedal and the 2 ends pistons, or if it came from the secondary master cylinder and the centre piston. That should have been the big red flag for me then and there. Sometimes you learn as you go, and you err.
I also replaced the new after market pads with the oh so expensive OEM's with no improvement... Every components of the rear brake assembly was apparently fine or at least not apparently defective, worn, bent... to my novice eyes at least.
I decided to buy a complete used caliper and support arm assembly on eBay from a bike salvage parts guy in Texas ( eBay ID
cyclestripper ), as a new OEM set is very expensive, especially not knowing if I really needed it, and it takes 6 to 8 weeks to get one here in Ontario... The assembly (complete with arm and ready to install) looked clean and in good shape in the post pictures, and it was only 60$ + some shipping cost! Not much to lose at this point, right?
Well, good news! I am back on the saddle and riding again. Problem solved!
There is no way my support arm was bent or worn out (I had already replaced it just in case, for no gain). What could possibly go wrong with the caliper body? It is basically a big chunk or rugged metal... The pistons and seals were freshly replaced, cleaned polished and lubed. The only thing left I believed could have caused the jamming are the 2 guide pin bolts allowing the caliper to slide and maintain its proper position under load... Well guess what, the shop that did the caliper service at the beginning of this saga charged me (50$) for those 2 new bolts as part of the service kit, but they never actually replaced them, and did not keep them when I went back asking for them. Bad bad business!
I am glad this is over! This has been a challenging and frustrating job for my mechanical skill set.
Thanks to all of you that helped me understand how these linked brake work and what to look for.
W.