Rear caliper not releasing

I'm pretty good with brakes but this one has me stumped.
The bike is a 81 KZ750E2. My rear brake caliper releases fine for a while but then starts to hang as I use it. The whole system is "sterile" as far as cleanliness and well bled. When it starts to hang up I can only get it to release by cracking the bleeder valve or purging the system somewhere else as far foward as the banjo fitting at the master cylinder meaning there are no obstuctions from the cylinder to the caliper.The slides are also clean and move freely. When it locks up the brake pedal has no play, it is stiff. So I put it to you guys, any ideas where to go with this one??

disconect the pushrod to mastercylinder from brake pedal.is the pedal binding up when it pivots?shoot some wd-40 or something inside the pivot for brake pedal.if that doesnt help id pull caliper off and check piston and seal,might be some trash in there somewhere.

I've taken the pivot point apart and slathered it with anti-seize, it moves freely. I've shyed away from dis-assembling the caliper but that seems to be the last step I can take.
Thanks for the reply.

Since you can open the bleeder valve and the caliper will then retract normally, that kinda eliminates the caliper as the source of the problem.

According to your description of the problem, and the steps you have taken, the logical conclusion is a problem with the master cylinder, something in the master cylinder is not allowing the fluid to return.

My guess is that you either still have air in the system, or the piston in the master cylinder is sticking. Based on the symptoms you describe, I think air in the system is the likely culprit.

Do a search of the archives and look at the number of people that have sworn that there was no way there could be air in the system after they have repeatedly bled the brakes, and later admitted that was indeed the problem

I hear what your saying. I think I'll try the endless bleed where you run a hose from the bleeder back to the resevour. Beats the hell out of taking the caliper apart. I'm 99 9/10% sure the piston in the master is sliding freely.
Thanks

did you clean out the spooge hole in the master cylinder? that cased the same problem in my yamaha. you just have to clear it out with a paper clip

There are 2 holes but one looks like it is just an indentation. Should that be open as well?

btchalice wrote:

did you clean out the spooge hole in the master cylinder? that cased the same problem in my yamaha. you just have to clear it out with a paper clip

terry?? is that a technical name??? spooge??...steve

fixer5000 wrote:

btchalice wrote:

did you clean out the spooge hole in the master cylinder? that cased the same problem in my yamaha. you just have to clear it out with a paper clip

terry?? is that a technical name??? spooge??...steve

yup, the spooge hole is right next to the whowhutsit,right below the thingamajig:lol:

I'm glad we're having fun with Terry's Spooge Hole remark but..... should that indentation be an open hole?
Thanks,
Thom

The answer is YES. It is a VERY small hole. You will need to use a small stiff wire (guitar string?) to open it up. And yes, it can cause the problem you have mentioned. I made the mistake of drilling mine open too large, and that will give you poor soft braking feel.
/

One possibility, if you have adjusted the rear brake linkage from the brake pedal to the master cylinder is that you have compressed the brake master cylinder to where it stays past the "spooge hole" and does not allow the pressure in the brake line to bleed off when you take your foot off the pedal...

I've seen it before, if people really try and adjust their brake to be super-sensitive.