Rear brakes

Hello everyone, my rear brake are stucking when its really hot outside.. even if I don’t touch it !! Rebuilt the caliper, piston move like a new one. But when I bleed the system, it seems like it has always some air in the line?!!  The Oem Hose are crimped with some “Plumber” collectors!!  My oil level is always at full..  when the caliper stuck, I open the bleeder on it and Im ready to go again !! 
thank you for your help
 

Sounds like a collapsed brake line to me. When you open the bleeder your releasing the pressure that’s a tell tale sign of a  collapsed line.

Does the air can cause that with the sun heat? A cold night will never collapse my brake .. so weird

What do you mean plumbers collector, is it an old oem stock brake line?
Let’s see a picture.

Check the tiny relief port in the MC is not clogged.

Animation made by R.I.P loudhvx, AFIAK


 
 
 

MC normal movment

 

MC bleeding movement

 

Yes its my oem brake system!! Dont looks like new bike with Bolt to master cylinder!! Some clamp to the oil bottle and master cylinder!! Thank you for the picture. I will disassemble the master.. cannot find some pieces for rebuilt rear master cylinder 

I agree with scirocco, I had a KH400 with same issue on the front (crack the nipple and good to go again) the return port in the master was blocked with crud.

Thank youuu I appreciate 

I'm another agree. Just had this on my ST on the front. After changing the top hose it wouldn't bleed and very small bubbles came out the feed hole. My suspicions were raised when it was impossible to back bleed.  Once the hole was cleaned out it was bled in 5 minutes. I had initially put it down to a collapsing brake line or old fluid.