Rear Brake Locks Up

[ejw24]

Guys so weird problem - while riding the rear brake locks up on it's own!  It first happened a few weeks ago while riding down the interstate.  The bleed screw blew out of the caliper so I bought a new caliper and put it on, bled it and rode another few weeks then it happened again.  So I bought a new rear master cylinder put it on and bled it.  Worked fine for two weeks until yesterday.  After riding for about 3 hours it locked up like I put the rear brake on but wasn't touching the pedal.  Caliper & rotor gets smoking hot.  Only way to get it to release is to crack open the bolt on the banjo brake line fitting to relieve the pressure.  Any thoughts as to what could be happening?  I'm at a total loss, THANKS!!

[Bones]

yikes, that's a weird one.

you technically replaced everything except the line, which may be the culprit, if it's like partially collapsed or even clogged or something (which is also weird in the first place). something is causing a real pressure build up and the rear brake is such a simple system, that the oine is all I can come up with. did you possibly out the spring back on the wrong way?

  Are you using DOT 4 fluid?

[ejw24]

yikes, that's a weird one.

you technically replaced everything except the line, which may be the culprit, if it's like partially collapsed or even clogged or something (which is also weird in the first place). something is causing a real pressure build up and the rear brake is such a simple system, that the oine is all I can come up with. did you possibly out the spring back on the wrong way?

  Are you using DOT 4 fluid?

Yes, using DOT 4 & the pedal/spring works fine.  I wouldn't think the line would be clogged as it bleeds ok but you are right thats the only part I have left to replace.

[DrDesmo]

Try adjusting the rear brake pedal free play at the master cylinder.  Your owners manual will have the instructions about how much movement should be in the system.

When it's not adjusted correctly it can cause the issue you're describing 

[ejw24]

Try adjusting the rear brake pedal free play at the master cylinder.  Your owners manual will have the instructions about how much movement should be in the system.

When it's not adjusted correctly it can cause the issue you're describing 

SWEET, thanks for the tip!!

[DrDesmo]

SWEET, thanks for the tip!!

No problem, I'll save you some flipping about in the manual too --

[ejw24]

No problem, I'll save you some flipping about in the manual too --

Re-bled the system and adjusted the pedal last night.  Keeping fingers crossed it works this time...  Thanks for the tip!

[ejw24]

Damn thing did it again today!  Rode it 3 times since adjusting pedal and bleeding the system. Locked up on the interstate doing about 80...Any other ideas on how to fix it?  Should I replace the line??

[devimau]

has the bike ever been crashed on the brake pedal side?

[RAT900]

No problem, I'll save you some flipping about in the manual too --

Assuming you backed off the plunger pin on the brake pedal (items 6 & 7) so it is not putting pressure on the master cylinder piston while in its static/idle state.

Assuming your pedal return spring is not broken or missing and is still there forcing the pedal to return to idle state

Assuming you did not permanently cook your rear caliper causing intermittent sticky pistons

Assuming the master cylinder piston is not hanging up intermittently

Assuming the line is not rubbing or pinching anywhere on its route

If all the above checks out and the rotor isn't warped and you are still locking up........... then spring the 150 bucks or so for a new master and caliper w/pads and replace

I just did that with my recent 2002 900 acquisition because the caliper was a dead lump...the pad backings were crumbling rust

A rear Brembo caliper and rear master for my bike were the cheapest parts I think I ever purchased for it

PM me if you want the name of a guy that deals real Brembo at good rates...if you go to eBay beware of the Thai counterfeit knock-off crap that is flooding the listings

If all of the above does not solve the problem then chop your right foot off or stop wearing your six inch Disco platform shoes when riding...the 1970's are over 

[Howie]

In rare cases, a rubber brake line can tear internally and create a flap that will work as a check valve, allowing fluid into the caliper but not out.  I'm not sure if this is a possibility on a steel brake line, I don't think it is.  I would try adding more than the 2 mm. free play and check the caliper for proper movement first.  I would also suggest not going on the highway at 80 MPH until you resolve the problem.  Some have had their rear caliper go up in flames.  If ya got the bucks, a new master, line and caliper is not a bad way to go.  For sure, if the caliper was badly overheated, seals are cooked.

[ejw24]

In rare cases, a rubber brake line can tear internally and create a flap that will work as a check valve, allowing fluid into the caliper but not out.  I'm not sure if this is a possibility on a steel brake line, I don't think it is.  I would try adding more than the 2 mm. free play and check the caliper for proper movement first.  I would also suggest not going on the highway at 80 MPH until you resolve the problem.  Some have had their rear caliper go up in flames.  If ya got the bucks, a new master, line and caliper is not a bad way to go.  For sure, if the caliper was badly overheated, seals are cooked.

ok I'll try putting more play into the pedal. A new rear caliper was the first thing I put on, would hate to have to spend another $200 for another one.

Thanks all for the advice

[Popeye the Sailor]

+1 on what Howie said-for the love of all that's holy, please stop test riding it at 80 on the freeway. You'll find when it locks up really good, you're trapped wherever it stops, assuming you don't go down.

And yes, if it fails the right way, it can catch on fire. When you're trying to pee on your bike on the highway to put it out, you're having a bad day. 

[DarkStaR]

And yes, if it fails the right way, it can catch on fire. When you're trying to pee on your bike on the highway to put it out, you're having a bad day. 

[IdZer0]

I have  no real experience in these things but just throwing some ideas out there:
- reservoir not too full to compensate for break fluid expansion
- caliper itself not touching the disc