Brand new RS685 lever and RS785 calipers. I installed the front with no trouble. Shortened line, bled, ready to go. The rear, however, was a problem. I shortened and bled the line same as the front. The first pull of the lever felt normal and came about half way to the handlebar. The lever would reset but the caliper would not. I had to wait about 4-5 seconds as the caliper slowly reset allowing another lever pull.
I have extensive wrenching experience and have used hydraulic brakes for a long time without ever seeing this issue.
Air in the line? I bled the system again. Same result.
Sticky caliper? Unlikely as they are new but possible so I pushed the pistons out ~4mm, wiped them and lubed them with mineral oil. Reset the pistons. Bled the line. Same result. Lever pulled in but caliper would not reset.
Now I totally purged the line. Drained the oil and injected brand new oil from the caliper bleed port up to the lever. Problem was I had to push on the syringe with ~30 pounds of force to get the oil flowing up to the lever. This is not normal.
Clogged line? Seemed to flow from lever to caliper but not caliper to lever. I detached the line from the lever and pushed oil from the caliper up through the line with no trouble. No excessive force needed.
Bad lever? Each pull and release of the lever rest quickly and oil level rose and dropped at the now exposed connection port just as I expected.
Called several local shops and best suggestion I got was that the tension spring on the brake pads could be too weak. So I switched the front and rear springs. No change.
The original line as it arrived from Shimano was mounted to the caliper and had a barb installed at the lever end but not connected to the lever. The barb had some kind of white seal over it apparently to keep the prefilled oil in the line. Looks sort of like white Teflon tape. The front line had the same thing on it. It was extremely thin and appears intended to simply burst open after installation allowing oil to flow through the line. This mysterious white seal is not mentioned anywhere in the installation guides I read.
When I pushed oil up through the line with a syringe I could see a little flap of the white sealing material flop open and closed like a heart valve. My theory is that this little flap was able to open back into the line allowing oil flow toward the caliper when the lever was pulled. But it could not as easily open toward the lever when the caliper tried to reset the line. This only occurred when the line was attached at the lever.
I replaced the entire rear hose with a new kit including new connecting bolt, olives and barbs (part SM-BH59-JK-SS). This had no little white seals on the barbs. Refilled and bled the line (again). Perfect. Brakes work just as they should.
This was an incredibly frustrating issue and perhaps my experience can save someone else a bit of heart ache.
If anyone has another theory as to what caused this I'd like to hear it. But replacing the hose fixed my problem.