Something I learned rebuilding Calipers/brakes on the old V4's, when you rebuild, remove the seals, and using a dental pick, scrape out the groove the seal sits in, it's amazing how much stuff can get in there behind the seal itself. I've had some pistons stuck so tight that I had to plug the banjo bolt fitting with a bolt, and use the bleeder screw as a grease fitting, pumping in grease at 1500 PSI usually pops the pistons out. Cleaning out that groove allows the seal to sit back where it's supposed to. Basic theory behind how calipers work is this: The seal is square cut, when the piston moves outwards while applying the brake, the seal "rolls". When you release the brake the seal tries to go back to square, this retracts the piston slightly. This action, combined with the fact that no rotor is actually "true", stops the pad from dragging during normal use. Now, if you get a situation where the fluid pressure cannot be released, or the seal itself is binding on the piston, or a slider pin is jammed, and you start to get a dragging caliper. Yes, a sticking guide pin can also cause a caliper to bind. When servicing, remove the guide pins, clean out all the gunk(using non petroleum based cleaner-actual brake cleaner works well-carb cleaner? No-so-much) And lube with a silicone base lubricant(Sil-glide was my go to for years-but most auto parts stores carry something branded as Brake grease) Regular grease is a no no, the grease attacks the rubber boots and components and causes them to swell and start to disintegrate, which of course, negates all the work you just did. Personally, when bleeding I use a Vacula brand air powered bleeder tool, but, I have one.... This tool allows me to completely drain and fill the system with no issues. While working on the calipers themselves, you can clamp the rubber hoses closed(so the system doesn't drain dry). I, obviously, have special nylon clamping pliers designed to do this also, they don't damage the hose or liner, Vice grips can be used, but gently, you don't want to crush the hoses inner liner. I haven't had need to service any of the master cylinders on a 13 yet, sort of looking forward to it....
We should do a webinar at my garage one day....who needs brakes done? Bring the parts, bring the beer...:tools1: