Brake lockup

1981 Kawasaki KZ750 H LTD 

I am running a newly rebuilt ZXR1200 master cylinder, stainless steel brake lines, new banjo bolts, newly rebuilt calipers, new FA68 EBC organic pads. 

I started this issue when I was riding up in the mountains with some friends, after coming down the mountain using a bit more brake than I should have, the brakes locked up in the middle of the road. I came to a stop and we had to take 3 grown men to push it out of the road. Once in a safer place to diagnose I saw smoke rising off the calipers and assumes the brakes got too hot for the organic pads that were on it and locked the brakes down. I ended up taking the calipers off and riding the 20 miles in backroads home. After that I rebuilt the whole system, blew out the stainless lines, put a rebuild kit in my master cylinder, replaced seals in both calipers, regreased slides and put in new EBC organic pads. It rides fine for another 500 miles, till today. After a short ride, 15minutes, zipping through the mountains, they tighten up again, not as bad as the first time but it would still not go anywhere. I was by myself luckily I was able to get it off the road. The brakes were definitely hot but should not be anywhere near hot enough to lock up like last time. I’m kinda stumped. After allowing the brakes 5 minutes to cool, they freed up and I was able to ride straight back home with only a rear brake and engine braking. One of my rotors have turned purple around the edges due to the heat. Does anyone have any suggestions? I can’t find any non organic pads that would hold up to heat any better, does anyone know if they make any? Thank you.

I should  also mention, when it happens the lever gets rock hard in the regular position, like not pulled in. 
After returning home today, I bled new fluid through the entire system and the fluid was honestly very similar to the new DOT3 fluid that I was putting in. 

Looks like one of the tiny release port in the MC is clogged.

 
 

My KH400 had the same problem many years ago, I used to carry a spanner to crack the bleed nipple and release the pressure until I got a replacement MC (was my only transport back then)

I would investigate the blocked port as detailed by Scirocco.  This is a classic issue when the master had crud it in.  Those ports shown are TINY.  

Thank you all, it seems everyone is in agreement that is the issue at hand. I recently rebuilt the master cylinder, cleaned it out good, took a bore brush down it and scrubbed it good and then sprayed it out with brake cleaner. I guess I didn’t get everything out. I’ll post a picture later to ask you all which hole I need to be concerned with, would it damage anything to poke the holes with a pick? 

Here is the photo, one big hole and one small one.

The possible tiny clogged hole in question is the tiny black spot. A single string of a wire or tooth brush should fit through the hole.

 

Also check your rotor for warpage since it was overheated.
Just lay a straight edge across the diameter.