I am a thoroughly qualified contender for World's Worst Mechanic. I can take the simplest task and turn it into a conundrum like nobody else. So I figure I can change my rear brake pads. Out comes the little pin, tap out the larger pin, take out the old pads, no problem. I expand the caliper and push in the piston with my tire iron just like Helge Pederson does on his video. The problem comes when I try to put the new pads in. The "piston" side. or the left side went in fine, but I had to actually remove the wheel to get the right side pad in, and now the wheel barely turns. You have to muscle the rear wheel (up on the center stand) to get it to turn at all. That aint right, right? Somebody suggested bleeding the brake lines, which I don't feel qualified to do. Any help?
It sounds like this bike uses a "sliding" caliper ( some people call this "floating" instead of sliding ) Anyway, a sliding caliper has a piston ( or pistons ) on one side only and the caliper itself must slide laterally in order for clamping force to be applied to both sides of the brake rotor. It sounds like you need to slide the caliper back to the position it needs to be in when new pads are installed. Often the pins on which the caliper slides get sticky and need to be lubricated. Usually some MOS2 grease is the choice for this BUT I hesitate to recommend this because BMW may have specified some other lubricant. Anyway, you need to slide the caliper such that the pads on both sides have proper clearance with respect to the rotor ( rotor = brake disc ). It is wiser to let a pro do the work on systems which are critical to safety, if you don't know what you are doing. Internet forums are no substitute for knowledge and basic competency, especially when there is no way to even determine whether advice you find on such forums is good or bad information. ( for example, I don't have a BMW 800 so maybe I missed giving you some critical info in what I wrote above ) It would be prudent to have your work checked out by a pro before you ride the bike on a public road. .
It is a sliding pin caliper. It looks like it is all of 1 piece but is not. Place the palm of your hand on the outside of the caliper and push towards the center of the wheel. There will be some resistance from brake fluid being pushed from a cylinder, but it will move the offside pads away, unless the slider pins are rusted up. To lubricate the pins completely requires removing the wheel and separating the caliper. Not needed on every pad change unless you find the pads dragging or resisting being compressed. Don't forget to press the brake pedal to get feel back before motoring off.
Can't get the off side pad to move? Got a lot of miles? sit in the rain?. Rusted up? If none of the above try pushing the caliper away. If it moves it will allow the wheel to spin. Pump up the brakes and see if they stick again or release. In a perfect world the pins should get lubed. I don't know what the manual says about it. They are pretty well sealed with rubber gaiters. I bet most DIY riders don't until they cause a problem. The most likely happening is that they won't release. A lot of pressure is exerted to squeeze them. None to release. Got a manual, it is not that hard. Take the wheel off. The bracket for the caliper has one pin into the caliper. The Caliper has a second pin that goes into the bracket. Unless you have some rust or grime they will pull apart exposing the pins. If not confortable have some one who knows do it.
Did you take the lid off of the reservoir and suck some fluid out ... When you push the piston back in the caliper the fluid has to head back through the system some times so much fluid has been displaced that if the cap is on you can not move the piston the entire distance needed.