What causes Front Brakes to LOCK-UP

[NightHeat]

Got an 04 GSX-R 750.  Took it out testing today and the front front brakes after using them for a tiny bit they begin to lock up.  To relieve the pressure I have to loosen the bleeder (the one at the master cylinder). I've bled the brakes over and over.  This is a used new bike that i put together over the winter.  Can't seem to figure it out.

Thanks!!!!

[Super Dave]

I've seen it where the resevoir was over full, and, when the fluid got some heat into it, the fluid had no where to go but to the brake pistons.  Worth a quick look.

[benprobst]

I've seen it where the resevoir was over full, and, when the fluid got some heat into it, the fluid had no where to go but to the brake pistons.  Worth a quick look.

+1+1+1+1+1

[Xian_13]

I have seen this, when people use the wrong brake fluid...
Like DOT 5 or such.

XIII

[Sobottka]

bad fluid?, maybe has a little moisture in it.

[NightHeat]

Well all is fine now that I dismantled the entire system and i mean the entire system 
It appears to me that the pistons in the calipers were sticking due to the bike setting for a long period of time without any fuid in the system (that would be my fault )  So anyways after a good flushing and cleaning of the system and by the way i did remove some pretty nasty thick goop behind the pistons. 

But I must say after getting everything back together and doing lengthy test ride everything turned out great and the brakes have a lot more feal, actually better then my woman's new 600.  But not better feeling then my woman. 

But in truth the fowling of the brake system was probably several factors all combined (old brake fluid, moisture, overheating, etc.).  So I would recommend flushing your system (master cylinder, brake lines, calipers) if you think your having some brake issues.  Then when you get it all back together and take it for a ride you'll be smiling just like me at a job well done.

[Jeff]

This is definitely a lesson for people though...

I've seen them lockup as Dave said, for too much fluid.  I've also seen them lock up for moisture in the lines (when water boils, it expands like 16,000 times or something ridiculous like that), and of course a sticky/cruddy/gunked up caliper piston.

[PolishPete]

Is this what happened in BikerBoyz?  During the drag race...something happens to the guys rear brakes and he crashed.  Anyone know what that was?  A lock-up?

[NightHeat]

I'm not sure if thats what happened to the BikerBoyz but I know that if I had been going any faster when the front brakes locked up on me, I would have known how it felt to be a rocket.