New bike's back brake stays engaged.

what should I check first if the back brake seems to not disengage properly?
Master cylinder or Slave?

Brakes bleed properly, pads within spec and line pressure feels right?

i.e.... The line jumps a little when pedal pressed hard or you can feel the pressure while holding the line... You can witness the pads squeezing but not retracting etc...

If above holds true, then may be time for a caliper re-build...

Rebuild the caliper and make sure you clean out the seal groove with a wire wheel on a dremmel. You could also take off the brake pedal clean it up and grease it. Mine was sticking bad and now moves like it was new.

The first thing to check is the brake adjustment screw.
Make sure the lock nut has not come loose and the screw has vibrated down and is holding the brake pedal from returning up all the way.
This little thing cost me a lot of time and money back in the 70's at the local repair shop and is one of the reasons I try to do all my own work now.

Good luck, JD

I rebuilt my rear caliper and installed a stainless brake line and new pads this summer and I had the same problem. The brake had worked fine before but it seemed like the new pads were too thick and the piston couldn't retract far enough to allow the pads to move away from the rotor. The old pads were still in spec so I put them back on and it's fine. Has anyone else had this problem?

Check inside the master cylinder for the two small holes in the bottom. If one or more gets clogged, the brake piston may not return and apply the brake.

A wooden toothpick works well to open up the holes.

Would use rear brake pushrod adjuster mechanism to add additional gap between end of the pushrod and master cylinder piston.

See part number 13 on image below.

A too small gap may soon disappear while riding and allow the brake to drag, which may increase heat and eventually lock up the brake.

Good Luck!

[Click on image to enhance view.]

Patton wrote:

Would use rear brake pushrod adjuster mechanism to add additional gap between end of the pushrod and master cylinder piston.

See part number 13 on image below.

A too small gap may soon disappear while riding and allow the brake to drag, which may increase heat and eventually lock up the brake.

Good Luck!

[Click on image to enhance view.]

I have the exact same problem, exact same rear brake assembly.

Part #9 in the diagram, the brake piston, is not descending. The brake push rod, part #13, is retracting just fine. I had to pull the whole assembly off the bike to discover this.

Why will the piston stick in the "engaged" position like that even after the pressure from the brake lever/pushrod is removed?

To qdude, if you have the same problem, it is hard to know for sure with all parts still on the bike because you can't really see that the part #9 is sticking when everything's still attached.

There are two holes in bottom of reservoir for transfer of brake fluid between reservoir and bore.

The larger hole is for supplying fluid into the bore, which fluid may be compressed by application of the hand lever.

The much smaller pinpoint size hole is the relief port, whereby pressure is relieved from the line and returned to the reservoir when the hand lever is released.

The small hole sometimes gets clogged, whereby fluid in the lines remains compressed regardless of hand lever position, in which event the brake remains applied.

Would assure the small hole is clean and clear.

Good Luck!

Both pedal adjustment/and freedom, and master cyl return vents are excellent suggestions. Another check,,, worn pads and extended piston w/build-up can cause poor piston return. The seals which seal pistons to caliper bores are actually square in cross-section and become "familiar" with piston.with pressure pistons travel, and seal deforms. when working correctly, with the removal of pressure the seal will retract piston slightly. try gently levering pads a couple mm's back into caliper, and watch for piston/pad motion while cycling brake pedal..Luck G