Brakes locked solid...a "Minties moment"

[kerryg]

Yesterday heading south on Southern Motorway, in the outside lane of 3 just before Gillies Ave and my bike grinds to a halt. I am right in the middle of the lane (there is a concrete median barrier but no pull-off area). Engine is fine, but the front brakes have locked solid. So...I can't push it ...and I can't ride it (bike stays stationary and rear wheel produces lots of cool smoke)....and meanwhile I have stopped traffic in all lanes, and chaos and mayhem ensues.... Finally 2 kind samaritans help me to carry the bike (dragging the rear wheel) across 3 lanes of motorway to the armco where I manage to disconnect the brake lines from the master cylinder as motorway traffic hurtles past about...ohhh...a foot away. Anyway got home alright but sort of spoilt my day.

So I pulled the front calipers off and disassembled them and found that there is thick grey sludge in them, under the pistons (but there is good clear fluid in the lines and in the master cylinder). And quite a bit of black gunk stuck to the pistons. I'm going to clean them up and reassemble them and replace any seals that look suspect and hope that that cures it but I'm a bit puzzled why it occurred. My theory is like this: one or more of the pistons has been stuck in the "brakes on" position, the rotor(s) have got hot and the heat generated has overcooked the brake fluid which has boiled and pressurised the system. Probably helped by the fluid being so crappy and possibly with water entrained? This theory might be supported by the fact that that the brake lever was ROCK hard (there wouldn't have been a millimetre of play) and that as soon as I loosened the banjo fitting at the master cylinder the brakes freed up completely.

Any opinions as to causes and solutions, especially if different to those potulated above,will be gratefully received. I'm kind of hoping not to be in such a situation again...ever!!

[vifferman]

So I pulled the front calipers off and disassembled them and found that there is thick grey sludge in them, under the pistons (but there is good clear fluid in the lines and in the master cylinder). And quite a bit of black gunk stuck to the pistons. My theory is like this: one or more of the pistons has been stuck in the "brakes on" position and the heat generated has overcooked the brake fuid which has boiled and pressurised the system. Probably helped by the fluid being so crappy and possibly with water entrained?

Yup, cooked sludge.

[Racey Rider]

Yep. Happened to me on the track once. Learn as you go sometimes.
Just lucky you didn't get killed with it happening on the motorway!

[Devil]

Yeah, scary place for something like that to happen.

[kerryg]

Yep. Happened to me on the track once. Learn as you go sometimes.
Just lucky you didn't get killed with it happening on the motorway!

It could have been a real disaster. It really could have caused a fatal crash. That section of the motorway has traffic moving at probably max 70-80km because of a speed restriction and merging traffic from Grafton. If it had happened a few kms further south, me in the fast lane....I hate to think..and how many of us are guilty of being a bit lax about renewing their brake fluid regularly??

Anyway I have learned a valuable lesson: change your brake fluids regularly!!!!....

[F5 Dave]

Yeah any debris sinks to the bottom part of the system. Your theory is most likely to be the correct one.

2 more things to check though. Confirm there is definitely play between the brake lever & master cylinder piston. A crook after market lever may close up the return hole & the system becomes sealed & any temp buildup will cause what you described. I guess some debris blocking this hole may do the same.

Check the rubber in the master cylinder isn’t ripped & the vent/s in the cap aren’t blocked with white powdery crystalisation (small channel leading to side of cap). This would also seal the system.

[FROSTY:Me]

you probably wont need to renew seals but seeing as ya have the system apart you should buff up the pistons and use rubberlube on all the seals.
When you put it all back together if its a bugger to bleed up try cracking the joints -it saves a heap of bleeding.

[kerryg]

Yeah any debris sinks to the bottom part of the system. Your theory is most likely to be the correct one.

2 more things to check though. Confirm there is definitely play between the brake lever & master cylinder piston. A crook after market lever may close up the return hole & the system becomes sealed & any temp buildup will cause what you described. I guess some debris blocking this hole may do the same.

Check the rubber in the master cylinder isn’t ripped & the vent/s in the cap aren’t blocked with white powdery crystalisation (small channel leading to side of cap). This would also seal the system.

Thanks for the advice Dave and I'll definitely check those points when I put it all back together

[kerryg]

you probably wont need to renew seals but seeing as ya have the system apart you should buff up the pistons and use rubberlube on all the seals.
When you put it all back together if its a bugger to bleed up try cracking the joints -it saves a heap of bleeding.

Thanks for the advice Frosty and I'll do that

[vifferman]

you probably wont need to renew seals but seeing as ya have the system apart you should buff up the pistons and use rubberlube on all the seals.
When you put it all back together if its a bugger to bleed up try cracking the joints -it saves a heap of bleeding.

... and one more thing: don't get brake fluid on your skin! It soaks in, it's not healthy, and it's annoying: first your skin is all shiny for a few days, then it flakes off...

[F5 Dave]

Not often the seals really need replacing unless they have been damaged. I use non aggressive plastic scouring pad to remove surface rust from the pistons, ideally you want to keep the plating on them or the will rust up faster.

Never noticed skin damage, but worth mentioning. Those latex gloves are virtually worthless but wander along to the local protector safety shop & get a box of nitrile ones which are mint green & far more resistant chemically & rip resistant (within reason). Only ~$40 for a decent box.

Obviously brake clean is great for cleaning the metal bits, but keep away from the seals.

Also when you pull the seals out there will be some more white crystal buildup behind the seals that you will have to carefully scrape out. any marks in the bore where water has corroded may be polished out V. gently.

[speedpro]

I manage to disconnect the brake lines from the master cylinder

to free it up.

This is a sure sign that the fluid return vent into the master cylinder was blocked, and as you also said that the lever had no play, it is almost certain that for some reason the lever was pushing the master cylinder piston in a bit and that was blocking the vent. I've seen it lots at bucket racing where guys have bought odd levers for their bikes. If you just jiggle the lever a little bit you should see the fluid flow back out of the vent just at the start of the lever movement. In combination with a lack of maintenence it's a disaster waiting to happen.

[kerryg]

to free it up.

This is a sure sign that the fluid return vent into the master cylinder was blocked, and as you also said that the lever had no play, it is almost certain that for some reason the lever was pushing the master cylinder piston in a bit and that was blocking the vent. I've seen it lots at bucket racing where guys have bought odd levers for their bikes. If you just jiggle the lever a little bit you should see the fluid flow back out of the vent just at the start of the lever movement. In combination with a lack of maintenence it's a disaster waiting to happen.

Hi Speedpro. Thanks for that. I'm going to very carefully check that the vent is working and that the lever is not somehow interfering with its operation. I'm quite sure that the whole system was pressurised, which implies a failure to vent so I think you're on the monery there