Front brake locked up. XS650SH

[jetmechmarty]

I was going down the highway and gradually unable to stay up with traffic. I got off the road and the front wheel was locked up with the front brake smoking. It took about 30 minutes for my bride to show up with a bleeder wrench and by then the wheel was rolling.

I recently replaced the master cylinder with an 11 mm. I rebuilt it. It’s been working well for lots of miles. Years ago when I put this bike together with my daughter it got new stainless lines from MikesXS. The outer jacket is deteriorating. For now, it healed itself.

The only time I’ve seen this before is when the vent in the master cylinder was plugged. I haven’t seen that issue fix itself.

Could this be caused by brake line failure? This happened after driving over five miles without touching the brake.

Length of lines? I won’t installing Chinese this time. “Fool me once” they say.

 

[GLJ]

This happened after driving over five miles without touching the brake.

I'm not a big fan of the cheap chinessem brake lines. I prefer a quality brand like HEL.
You need to figure out what the problem is. My guess and it's only a guess is the caliper didn't release all the way. Pressure built up from the heat and the MC did not release. Something had to cause pressure to build in the system while you were riding.

 

[jetmechmarty]

I'm not a big fan of the cheap chinessem brake lines. I prefer a quality brand like HEL.
You need to figure out what the problem is. My guess and it's only a guess is the caliper didn't release all the way. Pressure built up from the heat and the MC did not release. Something had to cause pressure to build in the system while you were riding.

I agree. I was surprised brake fluid didn’t squirt out when I opened the bleeder. It seems to me, the only possibilities are the master cylinder vent blocked or blockage in the line. The brake lever was very tight, no travel in it.

 

[Jim]

The brake lever was very tight, no travel in it.

That "points" to the master cyl vent hole I think.
If it were the line, you'd expect a little travel while the line pressurized up to the blockage point.

 

[jetmechmarty]

That "points" to the master cyl vent hole I think.
If it were the line, you'd expect a little travel while the line pressurized up to the blockage point.

Yes. It’s just very strange that it healed itself.

 

[5twins]

Normally, this happens if the fluid is old and has water in it. As it heats up, the water expands and that applies the brake. But you said the fluid is fresh? How fresh? Because the brake will release when a system with water in it cools down.

 

[jetmechmarty]

Normally, this happens if the fluid is old and has water in it. As it heats up, the water expands and that applies the brake. But you said the fluid is fresh? How fresh? Because the brake will release when a system with water in it cools down.

The fluid is only months old. I’ll be changing it just the same.
It was fresh in March. Last page of the thread. https://www.xs650.com/threads/reunited-with-xs650sh.63667/page-10

 

[Its been a long time]

Agree, 99.9% chance the vent hole was blocked. Only other possibilities are piston stuck or the floating caliper seized on its floating bush and rubbing the disc. More likely the master cylinder, I think.

Just thought of one more item to check. I have seen this happen when the lever free play adjustment screw is a bit too tight and is slightly pushing the master cylinder piston at all times. I'd double check there's a tad free play between the brake lever and piston.

Also seen this where a Chinese lever was fitted and it was slightly incorrect, again allowing zero free play at the lever.

Lucky that it wasn't a more serious event.

 

[Max Midnight]

Normally, this happens if the fluid is old and has water in it. As it heats up, the water expands and that applies the brake. But you said the fluid is fresh? How fresh? Because the brake will release when a system with water in it cools down.

'Fraid I have to disagree with the above.
Brake fluid is hydroscopic i.e. it absorbs water which reduces its boiling point.
Other than causing internal corrosion this isn't an issue until it boils and turns to steam.
Steam is a gas and therefore compressible and the movement of the m/c piston will trey to compress this before moving any brake fluid.
As a result there will be less movement of the pads and the braking effort will be reduced.
It will not cause the pads to lock on.

I would make sure that the caliper pistons are free of corrosion and are able to move freely and that the m/c piston is able to fully return.
As the symptoms seem to point to a heat relate issue (as the wheel freed up as the system cooled) I'd be inclined to a problem with the caliper rather than pressure in the lines keeping the brake on.

 

[Wagner73]

Err... hygroscopic.

Yes, I will throw in with the group pointing to a caliper/ line issue. It could be that the line(s) are internally breaking down a bit and debris is affecting piston movement,and/or a piston/seal issue.

I would think that if the return (vent) hole in the m/c was the problem, it would still be the same problem regardless of hot or cold at the caliper end.

 

[AUTOMAN]

'Fraid I have to disagree with the above.
Brake fluid is hydroscopic i.e. it absorbs water which reduces its boiling point.
Other than causing internal corrosion this isn't an issue until it boils and turns to steam.
Steam is a gas and therefore compressible and the movement of the m/c piston will trey to compress this before moving any brake fluid.
As a result there will be less movement of the pads and the braking effort will be reduced.
It will not cause the pads to lock on.

I would make sure that the caliper pistons are free of corrosion and are able to move freely and that the m/c piston is able to fully return.
As the symptoms seem to point to a heat relate issue (as the wheel freed up as the system cooled) I'd be inclined to a problem with the caliper rather than pressure in the lines keeping the brake on.

In keeping up with Max...

DOT 3 or 4 will be hydroscopic. USE DOT 5.
Slave(caliper) seized/cocked due to corrosion/wear.
Carrier(bracket) seized.

Slave piston is meant to retract by the rubber (radial) seal by 5 thou or so.
Pressure is meant to relieve via master bleed off hole.

As for healing themselves, a cut rubber piece when the rubber lines were installed onto the steel lines will cause a check valve that does this.
Time and use may free it up, just to occur again.

cliff

 

[jetmechmarty]

DING DING DING DING DING!

We have a winner!

Just thought of one more item to check. I have seen this happen when the lever free play adjustment screw is a bit too tight and is slightly pushing the master cylinder piston at all times. I'd double check there's a tad free play between the brake lever and piston.

I had the bike in the driveway, baking in the Mississippi summer sun while I was going over the bike cleaning, washing, etc. The front wheel locked up. Just sitting there in the hot sun, it locked up. I checked for free play and there was none. The lever was tight as a drum. I've not done that before.

I'm sure it's all better now. The brake released immediately as I removed the lever.

 

[Its been a long time]

DING DING DING DING DING!

We have a winner!



I had the bike in the driveway, baking in the Mississippi summer sun while I was going over the bike cleaning, washing, etc. The front wheel locked up. Just sitting there in the hot sun, it locked up. I checked for free play and there was none. The lever was tight as a drum. I've not done that before.

I'm sure it's all better now.

Excellent to hear the root cause was found. The outcome might have been much worse if it happened again.

 

[jetmechmarty]

Excellent to hear the root cause was found. The outcome might have been much worse if it happened again.

The brake was coming on like I was going uphill. It was getting steeper and steeper until I could smell the brake. It was really smoking!

 

[jetmechmarty]

Many thanks to all who gave the thoughtful responses. I appreciate every one of them.

I didn't mention this, but the highway I was riding had no shoulder. There are elevation changes and people drive stupid fast. The best I could do was get to the edge of the pavement then drag the thing to where there was space for a mailbox. I was unable to get to the left sidecover where there lives an 8 mm wrench. Doing so would have put my body entirely on the roadway. I called wife rescue service.

 

[Kevin Kruse]

Nice find, glad it wasn’t worse. Made me double check my lever screw..

 

[Wagner73]

Happy you found the root cause!!

 

[AUTOMAN]

So what was the root cause?
Brake on, covering vent hole?

 

[Wagner73]

DING DING DING DING DING!

We have a winner!



I had the bike in the driveway, baking in the Mississippi summer sun while I was going over the bike cleaning, washing, etc. The front wheel locked up. Just sitting there in the hot sun, it locked up. I checked for free play and there was none. The lever was tight as a drum. I've not done that before.

I'm sure it's all better now. The brake released immediately as I removed the lever.

View attachment 354223

This

 

[AUTOMAN]

Missing the point, so lever was removed and brake was free. Now no brakes?

What was the root cause? I know as to may others but to a novice is not clear why?

Not trying to be an ass, it's just for a newbie, he/she/it would not know.