Hi guys - Just back from 5 days in NC / TN on the Dragon, Parkway, and all the super tight curvy off-shoots, and Day 3 while rippin' down 215 all of the sudden the rear brake lever sank to the bottom! YIKES! No rear brake! Lasted that way the rest of the way to the bottom, with me just lightly touching it again from time to time just to see if it was back, and if it was boiling I didn't wanna make it worse, and certainly didn't wanna try to rely on it. I'm def not somebody who rides the brakes at all, altho the big girl is heavy so as y'all know it requires some serious squeeze when needed So I rode the rest of the way assuming fronts only which wasn't much fun ... buddies long gone At the bottom did a U-turn and went back up 215 to the Parkway, taking it easy again still fronts only, and at some point near the top the rear started working again. Cooled down? Dunno. Oh, about a month ago replaced the brake fluids, front and rear, and had bud help so we never drew air, kept it full the entire time, so no bleed necessary ... or so I think? Pads are worn but still plenty, I think. So, Day 4 brakes worked fine the whole day, no such collapse. Day 5 it happened again, but didn't last long before it came back good again. Bike's at 17k miles now, orig pads front and rear. Master cylinder seal issue? Stuck pad(s) Other?

I've done some reading, and from what I see the most likely culprit would seem to be a leaking master cylinder seal, but def not sure, and I'm def not a brake expert as I've never had any brake issues and actually my bud helping me replace the fluid recently was the first brake anything I've ever done on any bike I've owned; never owned one this long before! Just thought I'd ask here before getting into it. Any suggestions? Thanks for whatever ya got!

So basically the back brake was doing some sport riding. Ever got off the bike and checked for a hot rear disc? Disc burnt looking in color? The deal right now is to step on the brake hard, then off the pedal, now see if the back wheel drags or is easy to spin.

Drags, that says the quad rings need to be cleaned and/or replaced. With a metal scribe, remove any built up crust in the groove. Pistons and bore holes are never cleaned with sandpaper kind of material. Only cleaning is with a paper towel and brake clean.

Where spraying brake clean in the quad seal groove, air dry, you can still see the white crusty residue to keep scrubbing out you missed.

Quad seal sits in the groove of the caliper with the seal having memory and is at a static (||) upright position. When brakes are applied the quad is distorted at (//) at a slight angle, as the piston moves out. The crust build in the quad ring's groove keeps the seal from returning to its static (||) upright position, thus causing the pads to drag. That's what your looking for.

Check and report.

Hub - Forgot to state that while still up there and after the ride I did put the bike on its center stand and spun the wheel, and it spins free. Did not think to check for a hot disc, tho, darnit. Sounds like I need to do the 'press brake lever hard, release then check again' to see if the wheel spins free, since I suppose the pads could've been stuck then come free sometime after applying the brake ... ? Will try ASAP and report.

Thanks as always