Brakes lock up while parked.

Bike is '81 KZ1100 shaftie.
Calipers and master cylinder rebuilt 600 miles ago. Brakes work flawlessly while riding.
Yesterday, after a long ride I parked the bike for 15 minutes and then the front brakes were locked up.
I opened the front bleeder
(left side because that disc was 'warmer'. Actually just a random thing).
Fluid squirted out without me squeezing lever. Then brakes worked flawlessly for the ride home.
Problem repeated today.
Any ideas?

Park it in the sun??

Had the same thing happen to me about 30 years ago, rode my KZ1000 to work in the early morning, got on it in late afternoon to go home and just smoked the rear tire. Loosened the bleeder screw and I swear the brake fluid shot 20 feet!!

You have air in the lines that got hot and expanded, and the return hole in the bottom of the master cylinder is probably plugged (or your master cylinder was filled to the brim).

Yes, both times it happened it was parked in the sun.
I will disassemble the m/cyl, check it out and then bleed the system excessively.
Thanks!!

dont forget to check the brake line, it may have gone bad on the inside

All brake lines are new stainless flex hoses from Aeroquip.

Post edited by: FloridaBiker, at: 2008/02/05 08:01

Only time my brakes locked up on me, it was an issue with the brake handle. The adjustment screw had become tighter and was not allowing the lever to return back far enough.

I didn't know the sun affected the brakes. My bikes always used to be parked in the sun even at the end of the day when I was home and I would go out there and never have any problems. I think it was all due to the fluid level not being over the half limit.

kzwolfsr wrote:

I didn't know the sun affected the brakes. My bikes always used to be parked in the sun even at the end of the day when I was home and I would go out there and never have any problems. I think it was all due to the fluid level not being over the half limit.

Could be because you didn't have any air in the lines, air expands a whole bunch more than brake fluid does when it's heated

In my case (and probably the OP's as well) the cause was air in the lines being heated by the sun.

It's interesting that I had just installed stainless brake lines as well, same time as I converted to dual disks and silicone brake fluid (the one and only time I ever used silicone brake fluid).

Post edited by: steell, at: 2008/02/06 01:50

I am having a hard time buying into the sitting in the sun idea, you get more heat in the system when the brakes are applied.
looking at the air idea, then it should release when it cools off.
to me its going to be in the master cyl or in the caliper, possible piston sticking

The air in system is possible because it gets hot My M/CYL is black)from the sun, while the braking heat is at the caliper area.
It is definitely not the caliper sticking. WHY? Because I open the bleeder and fluid squirts out. Then the brakes work great, so I am sure the caliper is not sticking.
I will disassemble the M/CYL, bleed well, and then cross my fingers.

Post edited by: FloridaBiker, at: 2008/02/06 10:48

I had this problem on a dirt bike my nephew use to have.. Put new brakes on, swapped a master cylinder from a parts bike. Had a heck of a time bleeding the brakes, the guys on here got me thru it and it worked fine for a little while, but like your problem they seemed to freeze up when sitting. Never did figure that out, watching this to see what the solutions are..

FloridaBiker wrote:

The air in system is possible because it gets hot My M/CYL is black)from the sun, while the braking heat is at the caliper area.
It is definitely not the caliper sticking. WHY? Because I open the bleeder and fluid squirts out. Then the brakes work great, so I am sure the caliper is not sticking.
I will disassemble the M/CYL, bleed well, and then cross my fingers.<br><br>Post edited by: FloridaBiker, at: 2008/02/06 10:48

I had a car once that the front caliper was dragging, not much but I could feel it, got to the house and opend the bleeder screw, it shot out about 6 feet, the caliper was sticking causing the fluid to get hot and drag.
I was thinking just now, you changed the brake lines,is it possible thatthe line is too small and causing the restriction