Front brake locked up, I'm stumped

[glockman]

Just got back from a 3 day riding trip. On the first day of which my 08 YZ 450F front brake locked up. I had just replaced the front wheel bearings so I assumed they were not put in correctly and were bound up. I pulled the wheel to find they spun freely. I then cracked the bleed valve and it released and was fine for another 30 min of riding. Then the lever pressure started to build and eventually the wheel was nearly locked up again. Bleed a little fluid off again and repeat. It did this for 3 days. After 6 or so cycles I was low on fluid so I topped it off and bleed it some then started the cycle all over. I am stumped. I checked the caliper pistons and the pad aligning rods for grooves that might be hanging the pads but they were clean and smooth. It also wasn't on/off, the pressure would build in the lever and the travel would reduce until it was just locked up. Any ideas are appreciated.

 

[frieed]

Did you plug in the brake disable circuit? .... ... sorry, had to do it

 

[TRD270]

Is the brake dragging causing the caliper/fluid to heat up?

 

[glockman]

Is the brake dragging causing the caliper/fluid to heat up?

I don't think so. It spins freely after bleeding it so something has to be causing the brake to start dragging. I had the same thought but it would seem like that is putting the cart before the horse though.

 

[Spork]

If it's anything like a vehicle I'd be looking at the master cylinder. It sounds like it's not allowing fluid back when you release the brake. Could be something as simple as the lever not releasing all the way or crap in the reservoir.

 

[Unimog]

I'd agree that the master cylinder isn't releasing back to the point of letting fluid flow freely back into the reservoir. Maybe caused by bent lever or connection mechanism or something??? Could just be an adjustment at the master cyl also.

 

[rholbrook]

1: Have you had the front wheel off prior to problem occurring? If so, make sure you have the wheel spacer on the correct side. Sounds dumb but I have seen this happen.
2: Could you have hit the caliper on a rock? I did and it bent one of the caliper pins that it slides on. Similar issue as you are having. Mine was obvious when I pulled it to look at it.
3: Look at the pistons and make sure they are clean and no nicks or rust on them. Look at seal, is it leaking?
4: Bleed old fluid out, takes DOT4. Cheston and I did his last week.
5: I am assuming that the rotor is not bent.
6: Could be line blockage. May have to pull it and try to clear it with air.
7: Did you by chance add fluid to the master cylinder prior to it happening? There must me a little air or room in the master cylinder when you put the cap back on.

 

[glockman]

1: Have you had the front wheel off prior to problem occurring? If so, make sure you have the wheel spacer on the correct side. Sounds dumb but I have seen this happen.
2: Could you have hit the caliper on a rock? I did and it bent one of the caliper pins that it slides on. Similar issue as you are having. Mine was obvious when I pulled it to look at it.
3: Look at the pistons and make sure they are clean and no nicks or rust on them. Look at seal, is it leaking?
4: Bleed old fluid out, takes DOT4. Cheston and I did his last week.
5: I am assuming that the rotor is not bent.
6: Could be line blockage. May have to pull it and try to clear it with air.
7: Did you by chance add fluid to the master cylinder prior to it happening? There must me a little air or room in the master cylinder when you put the cap back on.

I had just replaced the wheel bearings. I was certain the spacers were swaped so I swapped them. The axle would go through so I swapped back. I'm thinking it is the alignment though. Pistons are clean and smooth, pad pin is straight and smooth also. I am going to flush and fill it anyhow since its 5 years old.


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[glockman]

After a little research on the interweb I found that the master cylinder has a return port to allow the fluid to bleed back into the reservoir when it gets hot. This port is blocked off in the first bit of lever travel. If the lever is adjusted wrong or the wrong lever is used it can block this port and cause the brake to lockup. I haven't verified yet but it sounds dead on to what I experienced. I'll post back when I verify this is the issue.