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Sticking rear caliper?

Ricknc

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2014 700 DCT, ABS,
I had the rear brake stick on me last week during a 150 mile ride. I noticed the engine was beginning to labor a bit pulling over to stop I pressed the rear brake lever with my foot and it went down to the pedal limit. I stopped with the front brake and didn’t see anything obvious but the rotor was really hot. The pads were still in and the fluid level in the reservoir was good. I rode it home using just the front brake. When I tried the rear brake pedal when I got home it was working again. I pulled the caliper and noticed a pad was fractured I cleaned the caliper and lubed the pins and contact surfaces,Cleaned and scuffed the rotor, replaced the pads. Flushed the rear system. Everything seemed good. I took it out for a test ride today and about 8 miles into my ride I felt the engine lugging again. I felt for the rear brake pedal and again it sunk to the limit. Rotor was hot . I limped it back home.
I am going to pull the caliper again and strip it down to see if there is any signs of the piston sticking. I have dealt with and repaired sticking brake calipers on Cars and trucks for decades. On this bike the brake pedal sinking to the stop has got me perplexed. Has anyone here encountered a similar situation? Am I missing anything obvious?
 

670cc

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The rear brake master cylinder could be the cause of the caliper sticking. I may be wrong but I seem to remember there is a tiny valve the allows pressure relief in the line when the lever returns to relaxed state. If the valve is plugged or sticking, over the course of multiple brake applications, the pressure builds up in the caliper and locks the brakes on.
 

Ricknc

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Thanks Greg, That makes sense, I had never heard of a seized caliper affecting the master cylinder. I don’t see any check valves looking at the parts diagrams. Nor does the 2014 and up have a separate proportioning valve, If it does it is built into the ABS module. In the morning I will ride the bike again until/if the brake seizes again. I will open the bleeder and see it the brake pads retract any. If that works I will try reverse flushing the system.
 

DirtFlier

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Yes, it's just a tiny hole in the floor of the master cylinder often imperceptable to the naked eye. Whenever I do my annual brake fluid change and bleed, I wipe clean any crud from the bottom of the reservoir.
 

Ricknc

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I rode again this morning and after a bit the brake started dragging again. I opened the bleeder and got a good squirt of hot fluid out of it,the pads immediately released the rotor. I will pull and disassemble the master cylinder in awhile for a good cleaning ( I will do the caliper also) followed by a thorough flush of the lines before reassembly.
 

Ricknc

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Opened up the master cylinder and there was some congealing on the reservoir side and some debris on the pressure side. It cleaned up easily enough , The springs seals look good but I ordered a new overhaul kit anyway.
The caliper obviously got very hot, there was transfer of some seal material to the piston, so the seals are no good. I soaked the piston in lacquer thinner for it bit and all the rubber dissolved from the grain. I cleaned up piston on the buffing wheel. It moves smoothly in the bore so I think I am good to reuse it. New seals on the way.
I flushed out the rest of the lines with DOT 4 in a bleeder pump.
 

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DirtFlier

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One thing to keep in mind is that water is heavier than brake fluid so it'll sink to the lowest point in the system, which happens to be at the brake caliper, so corrosion at the caliper pistons is caused by internal sources not water spray from the outside. This is another good reason to replace the brake fluid every two years, clean the inside of the brake reservoir, then refill with new fluid and bleed the system.

Using that policy, I've never had to remove the pistons from a brake caliper for cleaning.
 
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