2012 EC300R Rear Brake Problem
 

I've been having a problem with my rear brakes for about two weeks now and it's frustrating me to no end. Hopefully someone here can help me out. My rear brake piston wont return after being pushed out, causing the rear brakes to drag. I've rebuilt the MC, checked the brake line, and checked the caliper seals.
I've narrowed it down to something in the MC. It's not the brake line, since I can push in the rear piston with just the MC detached. The only thing I can think I may have done wrong is put in one of the MC piston seals backwards (both are cupping in the direction of compression), I figured the one on the bottom is a secondary seal and would cup in the same direction as the first. Any help would be greatly appreciated.

Do I understand correctly that if the master cylinder is attached you can't push the piston back in? It goes in only when master cylinder is detached? If so, then probably it can be the seal as you noted. Did you also take the rear piston out and clean it and also the piston seal? They get really dirty and even if you can push the piston back in, the brakes would still heat up.

Yes, you are correct. I'll try to swap the direction of one of the seals to see if that's the problem later on. I did clean the piston and it's seal, still no luck though. The only other thing could be that the port to the MC reservoir is blocked. Most brakes have two of such holes, but this one seems to only have one, or is it at the bottom of the reservoir?

Well, I finally figured it out. It turns out the cupped washer (not sure of the technical term) that goes above the circlip on the MC was in the wrong direction. The correct direction would be for the cup to bulge away from the piston, not towards. It seems it cupping towards the piston positions the piston such that it blocks the hole between the cylinder and reservoir (forgot the technical term...)

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Click as you take apart.

I agree, but I bought it that way. I sure wish the parts diagram had piston shown, may have saved me some headache.