http://forums.d2jsp.org/topic.php?t=70923155&f=27&o=0
That is tge original thread which there is some details.

So I recently aquired this bike, with a cracked piston. I had the bike bored out, put a over sized carb on it, stuck a bigger cam in it, staged the clutch and put a thunderhead performance exhaust on the bike. Runs like a champ.

Here's the thing.This bike has the dual caliper/rotor front brakes. I have bled, and bled, and re bled the brakes. I can get it to havw decent pressure but it bleeds off within a few hours and I have no brakes again. Im not losing fluid. The system is a pain in the ass, to say the least, to bleed. I've taken the calipers off and pressure tested them and they held. Lines have no visible weathering and the master cylinder never has bubbles in it...

My question.

What the fuck.

This bike has a open recall for brakes which the dealership will not honor duel to performance upgrades (yet I have stock brakes). I'm at a total loss. Any help?

More info about what the recall was in other thread.

cant you bleed the master cylinder on those? also im assuming you turned the handlebars to the left to keep the master cylinder the highest point. how does the brake lever feel when you have the brakes working good? can you hold the lever in and it wont keep going down on its own while you are holding it?

You can bleed the master cylinder. With the dual caliper set up its actually better to put the bike on a srand and have the weel straight, so the calipers are even. The lever feels very good when bled but the system has stopped bleeding as of this morning. I can pump allI want, no change in pressure, no change in fluid level.

could be a bad master cylinder then. probably the plunger has a cracked or broken seal in it.

That is my best guess I was just hoping someone could confirm my suspicion. Thanks.