Need some front brake help
 

I picked this bike up a few months ago and it needed quite a bit of work. I've pieced it back together, everything except for plastics. The only issue mechanically is, I am having trouble with the front brakes. The other day I took it for a ride and within 30 seconds the front brakes were locked solid. I let it sit for about 5 mins and it would roll freely. Got it back home and thought maybe the caliper was sticking causing it to heat up and expand because it was smoking. Pulled both calipers apart, cleaned them and put them back together. Even used a vacuum pump to make sure all the air was out of the the lines. Took it for a ride tonight, rode around the block a few times, maybe 5 mins and everything was going great. Thought I had fixed it. Pulled in front of the house, pulled the clutch in and came to a near screeching halt. Locked again. Lever is hard as a rock. Let it sit about 10 mins and it's fine. Any ideas what's going on? Could it be something with the master cylinder?

dirt on the piston ... ! happened to me on my other bike... you should rebuilt it. remove the galiper.. take it appart.. clean the piston and put everything together.. so take the piston out of the galiper use a air compressor.. but be VERY carefull not to get hit by the ppiston.. i almost got KO with it LOL ...

That's what I did, took them completely apart, pistons and all. Cleaned everything. Even put all new fluid in.

Well you need to put new fluid :P ..

I did...

It must be in the master. Break it down and inspect everything really well. The problem should be apparent since it sounds like you have a good idea of the basics.

It's just making me so mad because when it's cold, it works fine. I can pump the lever 100 times and it always releases. But after riding, that's not the case.

I had someone suggest that maybe one of my rotors was warped, causing too much friction in turn causing the rotors to expand..but wouldn't the pistons just retract back in if that was the case?

best way to find the problem... remove and rebuld the two galiper.. take 1-2 hours if you take your time.. (at same time you know they will be perfect condition after) ... if its still doing it the MC is the problem ..

Well I took the MC completely apart and cleaned it all. Put it all back together, bled the brakes again took it for a ride. Again, worked good for about 2 miles then brakes tightened up. Lever was getting very firm. Luckily I was thinking this time and took the wrench for the bleeder nut with me. Cracked open each side and all was good again. Came home. I'm thinking I should just find a used MC and try replacing that first. I can't see both calipers sticking at the same time.