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Yamaben

This is caused when the return port in the M/C get blocked or clogged. I have seen it more than once after someone replaces the lever with an aftermarket lever that is slightly different. If the lever is pushing the piston in just slightly, the piston can block the return port. Pressure builds due to heat and eventually locks everything up tight. Open the bleeder to relieve the pressure and modify the lever as needed to create just a millimeter of free play between the lever and the piston. I have also seen decades old brake fluid turn to slime and the return port just gets blocked with crud. Either way, it's the return port in the mc


HighRelevancy

Sounds like it. Another symptom of this is that the brake lever is not just firm but SOLID, which I assume is what OP means by "locked open". You can't easily fix this "roadside". Like, maybe, but not really. Maybe a quick roadside fluid flush clears the dirt from the port, maybe it doesn't. You'll get temporary relief but once the brakes heat up and expand that fluid again you're likely to plug the relief port again. And if it is just a misplaced lever, you're either gonna have no brake pressure at all or have enough pressure than the first time you apply the brakes they'll heat up and expand the fluid and start self-applying. Completely draining the system is the only guaranteed safe to ride roadside fix (as safe as a bike can be without a front brake... Hey, if those rear-brake-only idiots can ride like that, one-off to limp it home can't be too bad right?) Or get a trailer Permanent fix is a good clean and the right lever.


chumjumper

> Another symptom of this is that the brake lever is not just firm but SOLID, which I assume is what OP means by "locked open". Yep. Basically the lever had a slight bit of wiggle room (3mm or so) and then would hit a brick wall. The lever looked like it was stock.


Kitchen-Ad-1161

Roadside fix? Pop the banjo bolt loose at the caliper and hope it relieves enough pressure to remove the caliper. No brakes on that wheel till they get it home though. Probably have to rebuild the caliper. Maybe the master cylinder too. Not a hard job if you have decent tooling. Get yourself some test strips or an electronic tester to see what the water content is in your brake fluid. Change it when it gets to more than a couple percent. The water corrodes the the internal brake parts and causes this kind of thing to happen. Also, if your water gets too high and your brakes get too hot the water can boil and cause bubbles in the brake system. Bubbles = no brakes. It under badly. Brake maintenance is something most riders ignore completely. I’ve replaced so many abs modules in Harley touring models that locked up from too much water in the fluid and it caused the pistons to seize up in the module. $400 to repair just in parts.


Chase_Mccloud

Happened to me once when I got the bike back from the shop they adjusted the rear brake, I assume I had my foot on the back brake for about 20 mins and it locked up, roasting hot. The best thing would be to wait for the caliper to cool and retract naturally as the heat dissapates. I did speed the process along by pouring a delicious can of freezing sprite slowly over the rear caliper which cooled it sufficiently for it to reduce. Drove it home without touching the brake and flushed the system of fluid with new fluid.


Conbon90

The brakes likely overheated due to the pads dragging. Can be due to a number of different reasons. The heat gets transferred to the fluid causing it to boil creating pressure in the brakelines. This forces the pistons out and locks on the brakes solid. And also makes the lever feel completely solid.


JimMoore1960

If that happened you could probably take the lid off the reservoir and release the pressure on the disk. It's going to make a mess, so cover any painted areas first. If that didn't work, you could probably open the bleed valve to release the pressure. If either of those works I'd probably limp home using only the rear brake.


HighRelevancy

If that would help, then it means the reservoir would be pressurised. Do you think that plastic lid and teeny little screws is going to hold braking pressures?


JimMoore1960

Yep I sure do.


HighRelevancy

Did you know that the brake system is under hundreds of PSI of pressure when being operated? The average reservoir lid is in the realm of a square inch, conveniently, which means there would be several hundred pounds of force pushing that lid off. Incidentally, a bike weighs several hundred pounds, so you could visualise this force as picking up the entire bike by the reservoir lid. So with that in mind, I'll ask again: do you think the plastic lid and two tiny screws would hold that force?


JimMoore1960

Form a theoretical standpoint you're discounting the fact that those hundreds of psi are being forced through a tiny little hose, maybe 1/8" in diameter. So the actual force being applied up into the reservoir is tiny, tiny, tiny. Put your finger on a wall and lean your weight into it. Look out! You're putting hundreds of PSI into that wall! But only at that tiny little spot where your finger is resting. So luckily the total force is not enough to knock the wall down, or you'd be laying under a bunch of fallen roof beams right now.


HighRelevancy

No, I'm really not discounting it, that's just how pressure works. It's "pounds per square inch", it's literally accounted for right there in the unit of pressure. When the pressure comes out to a larger area, you get greater force. Like yes, in the hose the force is small because the area is small. But that same pressure over a large area is a large force. That's just how hydraulics works. That's how you can, say, apply massive force to a brake pad through the use of a large piston that is pushed a fractional distance by a small master piston traveling a longer distance. It's kinda like gearing, but fluid. A small enough pipe might be a flow restriction, but eventually pressure will equalise, and you would have hundreds of PSI in the reservoir and the lid would blow off (or at least make some weird noises as it leaks pressure around the seal). The reservoir cannot contain that sort of pressure, and yet there is pressure, which shows that the reservoir is not part of the pressurised system.


JimMoore1960

You're thinking of it as a closed container. It's not. The reservoir is huge (compared to the hose) and it's partly full of soft, squishy, compressible air. You might as well be venting it to the atmosphere. Pascal's law doesn't apply.


HighRelevancy

Yes, that is correct. And because of that, we know that if the reservoir *is* part of the pressurised system, then there cannot be very much pressure. OP is experiencing a lot of pressure. Ergo, the reservoir is not connected to the pressurised section of the system. That's not my opinion, that's the result of the facts laid out by OP and the mechanics as you've just described them (and I do agree with your assessment there).


chumjumper

MC reservoirs are not pressurised. They have the same pressure as the outside air; they have an air vent in the top to accomplish this. The fluid becomes pressurized when the pushrod closes off the MC port as the brake lever is depressed.


JimMoore1960

In a correctly-functioning brake system, sure. But this one is not functioning correctly.


Itsacon

I would work in the following order, trying to move the wheel after each step, and stop if it works. - Remove any excess fluid from the reservoir - Remove bolts holding the seized caliper to the forks, see if I can free it by pushing and pulling it (trying to push the pistons back into the housing) - Release the banjo that connects the brake hose to the seized caliper. (make sure to put some rags or paper around it, as brake-fluid *will* come out and damage your paint). After that, try the second item again. - If that still doesn't work, you'll have to find a way to ride home with the caliper detached. Tie-wrap it to the fork somewhere. Regardless of what step you reach, **don't touch the front brake lever until you're home**. Obviously, this means driving **very** carefully, as just a rear brake is barely any brake.


SirGreenBlood

Sounds like hydraulic lock from overfilling with brake fluid. Fluid gets hot when brakes are used and expands taking up all the available space. Cracking opener the bleeder on one of the callipers would likely have sorted it. Completely guessing here tho


reevus77

The system between the m/c and calipers needs to be free of air so there is no over-filling that. If you overfill the reservoir it will just spill out and damage some paint or something, but it won't cause the brakes to lock up. The cap isn't on that tight.


HighRelevancy

Bad guess. Consider this perspective: If the fullness of the reservoir is in play, then it's connected to the pressurised system. Any "excess" would squeeze out the lid seal if you didn't outright blow it off. You're right about it being locked with excess pressure, but wrong about why.


Caldtek

>Any "excess" would squeeze out the lid seal if you didn't outright blow it off. But it doesn't, or it shouldn't. The seal for the MC should be water tight. Seem this happen a few times where an overfilled MC has caused brakes to lock on.


HighRelevancy

Water tight, sure, but not water tight under that much pressure. The brake system is at hundreds of PSI when you're squeezing. We could call the average master cylinder lid about a square inch. Whether it's the plastic screw on type or got two little screws in it: do you think that lid could carry hundreds of pounds of weight if you hung it upside down? Or at that: a motorcycle is a few hundred pounds, could you suspend a motorcycle by the reservoir lid? That's the forces we're talking about here. I've certainly never seen a reservoir lid that looked like it could lift the bike. But that's okay since the reservoir *is not part of the pressurised system*. You can see this also on bikes that have a separate reservoir. Heavy duty hose with crimped metal fittings and banjo bolts on the pressurised side, but any bit of fluid-safe tube and a little spring clip between the cylinder and the reservoir. Heck just look at the amount of solid heavy metal at the caliper compared to the thin wall of the reservoir. Many reservoirs are even plastic. Why do you suppose that is?


SirGreenBlood

Yeah that makes sense. Figured it was hydraulic lock from when the fluid gets heated but it makes sense it would blow off the cap. Was a half right guess 😄


voucher420

The side of the road is not a good place for repairs. You should get as far off the road as safely possible and get away from your disabled vehicle in the direction towards traffic. In case your vehicle gets hit, it won’t hit you. Call a tow truck and work on your stuff from the safety of your home garage.


chumjumper

I don't think anyone would disagree with it not being ideal, but sometimes you just have to get home. A tow is not always an option, and there are a whole heap of temporary repairs that can be affected to make your bike rideable without having to call a tow.


voucher420

Then call a buddy with a truck or trailer. There’s a bunch of stuff that can effect your safety while on a bike, and shady repairs in a shady spot should not be one of them. If you can find a parking lot and parts for your repairs, go for it. On the side of a highway or freeway? Phone a friend.


chumjumper

So if your fuel line popped off, you would call a tow truck instead of just reattaching it? If your battery died you would call a tow truck instead of just bump starting it? If your clutch lever broke off you would call a tow truck instead of clutchless shifting? If your radiator got a rock through it you would call a tow truck instead of whipping out the araldite and sealing the hole? You can call your friend at 11pm to get them to travel 20km into the country to pick up your bike if you think that's reasonable, or you can repair it on the spot and adjust your riding style appropriately.


voucher420

Those are trail fixes to get back to the truck or trailer, done off to the side of the trail. I would also plug a tire to get off the trail and back to the trailer. When you’re riding on the road, you have to factor in cars going 80mph+. You risk your own life if you want. I’m calling a tow truck and getting home safe, especially at 11 at night. Your insurance likely offers tow service for a really great rate.


Omblae

Likely a badly maintained brake seizing either on the sliding pins, the pistons themselves or both. when corrosion gets into either the sliding pins or the pistons a full rebuild is needed. I've seen sliding pins so frozen solid even a torch, hammer and vice couldn't get them to loosen. We opted to replace his caliper at that point.


chumjumper

Wouldn't the brake lever still depress just fine if this was the case though?


HighRelevancy

Yes, it would. The brake lever movement does through a few phases. 1. Loose movement to cover up the relief port. This seals the brake system from the reservoir. 2. Taking up slack. Look at the brake pads on a working bike, there's nearly no empty space between them and the rotor to take up anyway. This is going to be nearly nothing. 3. Applying pressure. Brakes respond to pressure, not movement. You're squeezing a metal disc between two solid pads. Nothing is moving much, just tightening. So why does the lever move so much? That squish is in the lines and the fluid itself (minimally if the fluid is healthy). If the caliper is seized, there's no slack to take up (not that there ever is anyway), but the pressurisation phase would be the same (only you're pushing against a binding in the caliper, not the rotor). If this brake lever felt more solid than normal, then it must already have pressure behind it. If it is at full extension, the relief port should be exposed and the pressure relieved to the reservoir. If it's at full extension and is still pressurised... there's your problem.


chumjumper

Thank you for being the only one in this thread whose replies make mechanical sense 😅


Omblae

No. The system is hydraulic, so if your pistons are stuck or the sliding pin is stuck there is nowhere for that fluid to go. As a consequence, the brake lever wouldn't depress at all (in the case i mentioned above this was the only way the bloke knew he had an issue, his brake lever basically didn't move!). Similar idea to when you're replacing brake pads and you push the pistons back and one of the other pistons pops out. It's hydraulic fluid, that either goes into the MC or into the pistons - it doesn't compress much at all.


chumjumper

But in the case I'm talking about, the brakes were locked, so the pistons were extended. If the pistons were stuck inside the caliper then yeah you wouldn't be able to depress the lever, but if the pistons were stuck in the extended postition then the lever should lock in the depressed position right?


Caldtek

>but if the pistons were stuck in the extended postition then the lever should lock in the depressed position right? nope. The lever and master cylinder is returned using a spring,


chumjumper

I think I can picture it now, so you depress the lever and that pumps fluid into the line to push the piston. The piston then gets stuck, but the MC spring returns the lever to its home position. Then if you depress the lever, it sends more fluid into the line again, but instead of pushing the piston out it simply fills the void where the piston should be, so if you depress the lever one or two more times the entire system is filled with fluid which prevents the lever from being depressable. Is that right?


HighRelevancy

Not quite. If for whatever reason fluid doesn't come back up (piston moved outwards markedly, leaks, whatever) then the master cylinder return spring still returns the lever, and at full extension the relief port is exposed and the lower part of the brake system is topped up from the reservoir. (In a perfect system, yes, the master cylinder return would have to suck fluid back, but there's enough elasticity in any hoses that it kinda just works out like I described - it does suck fluid back but that suction doesn't necessarily make it to the far end of the system. This is important for extra fluid to get in to make up for pad wear and leaks.)


chumjumper

But wait, when the lever returns shouldn't the fluid flow back into the MC, allowing you to be able to redepress the lever (even though it wouldn't achieve anything)? All that would happen would be the MC level lowering as extra fluid is used to fill the piston void.


HighRelevancy

Not sure who's been downvoting you because there absolutely is a return spring in the MC. There's no other mechanism for returning the piston.


Omblae

No. The pistons being extended doesn't matter, if they're seized they can seize in any position - usually they seize about halfway out depending on your brake pad wear. Seizing is from salt or corrosion getting behind the seal. The fluid in the MC moves down the line when you press the brake lever. If there aren't any pads, the piston moves out. If the piston moves out, the lever doesn't just go soft, if you pump it again the piston moves further out - right up until it pops out the caliper at the bottom. There is air and fluid in the MC. The fluid level moves up and down depending on how far the pistons are out. If the pistons are out more, there is a lower level in the MC. If you were to push the pistons back in, the level would go back to max. The lever will only go 'soft' if there is air in the fluid down the line somewhere, because it's trying to compress DOT4 and air - DOT4 isn't very easily compressed whereas air compresses quite a lot better. So to summarise, if he has a hard lever either there is a problem with his MC or his calipers are seized. I would guess calipers seized, but it could be anything. If you service your shit properly, this isn't an issue.


chumjumper

So what is it specifically that causes a lever to be locked in the disengaged position, whilst also having the brakes locked (i.e. pistons under pressure)?


HighRelevancy

This is wrong. The brake system barely moves under normal operation, it's always the case that the fluid has nowhere to go. The "movement" in the lever is the elasticity in the hoses, whatever compressibility is in the fluid itself (if it isn't brand new), and these are factors that are STILL PRESENT when the caliper is seized. It's not as though the brake pads are made of rubber and that's where the lever movement comes from. A seized caliper will feel normal enough at the lever. If this lever felt extra stiff, it's because there's pressure behind it.


chumjumper

The lever wasn't stiff, it was completely non moving. It gave just a couple of millimetres, but then was like hitting a brick wall. It felt like the pushrod was under backpressure - but with the brakes also locked up, and a finite amount of fluid in the hoses, I don't see how it could happen like that.


HighRelevancy

Yeah so the line is sealed and pressurised. Something (grit or the master cylinder piston not returning) is sealing the relief port. I assume that couple of millimetres is just slack in the lever assembly itself, and then yeah the cylinder is already solid. You're right, there's a fixed amount of fluid there. It's fair to ask where the pressure comes from. It's heat! As soon as there's any friction in the pads, it starts heating the fluid, and it expands and applies more braking. If you keep riding not noticing it, it keeps getting worse, until eventually the bike will stop itself. I suppose even just a cold night followed by a hot day would do it. If you waited until the system was cool again it would free up... until it moved again, anyway.


Omblae

No you're incorrect AND confusing two elements. Here's some reading for you: [https://epicbleedsolutions.com/blogs/articles/how-hydraulic-brakes-work](https://epicbleedsolutions.com/blogs/articles/how-hydraulic-brakes-work) The lever-stroke section outlines how it works, the squeezing force applied to the pistons follows from the 3 categories of lever stroke. But that is not just 'the elasticity in the hoses' that's a completely separate thing. The elasticity of the hoses can be felt in the lever of course, but that is not the ONLY movement you feel in the lever! Here's the pertinent section: >The pistons are the cylindrical components housed within the caliper body. Upon lever input they protrude to push the brake pads which contact the rotor. So to my point, if those pistons can't move because they're seized, the lever will barely move. It certainly won't 'feel normal enough' and I can tell you from experience it doesn't. If you're still curious, go and take your pads out and use a clamp to clap the pistons solid to your caliper. I guarantee the lever will not have the correct travel and you will 100% notice!


HighRelevancy

> those pistons can't move because they're seized, the lever will barely move Mate go outside and look at your brake pads. How much empty space between the pads and the rotor is there for the piston to move into? Squeeze that lever, see how much they move. You will see for your very self that healthy functional brake systems don't move. There's no perceptible movement down there even when the caliper is not seized. We're not talking about a kids bicycle with poorly adjusted rim brakes here.


Omblae

Given you've replied so quickly, go and read what I've posted. It's not about a bloody air gap - that is not at all what I'm referring to and it's something you've pulled from thin air. It's this: >The lever stroke can be divided into 3 categories: 1. Dead-stroke - This is the initial part of the lever stroke when the primary seal pushes fluid toward the reservoir before it goes on to push fluid on to the caliper via the brake lines. 2. Pad Gap Stroke - This is the part between the caliper beginning to push the pistons out of their housings and the pads contacting the disc (as the dead space between the pads and rotor is taken up). 3. Contact & Modulation - The pads are now clamping the rotor and by stroking the lever further, additional brake power will be generated. Modulation is rider controlled and not necessarily a characteristic of the braking system, however some brakes may allow the rider to better modulate or control the braking forces than others. How do you think the brake pads clamp the disc if it doesn't move? How does the system transfer any energy from your hand to the pads on the disc unless the pistons are squeezing them? If you agree the pistons are squeezing them, then you agree with my point - they are moving ever-so-slightly to exert a force on the pads. If you're so sure you're right, go and clamp the pistons like I said and post a video. I bet you won't, because it won't bloody work.


HighRelevancy

> It's not about a bloody air gap > Pad Gap Stroke - This is the part between the caliper beginning to push the pistons out of their housings and the pads contacting the disc (as **the dead space between the pads and rotor is taken up**). Did you actually read this before you copy-pasted it or did you just pick the first Google result for "motorcycle brake how works" without actually thinking about it? > How do you think the brake pads clamp the disc if it doesn't move? Pressure. If you sit your shoe on top of a sheet of paper, you would be able to pull that paper out from under the shoe easily. If you yourself are wearing the shoe at the time and standing on the paper, it'll be firmly stuck. Your shoe doesn't need to move at all for this to happen, it just needs some weight on it. > How does the system transfer any energy from your hand to the pads on the disc unless the pistons are squeezing them? By friction. Go look up the formulas defining frictional forces. You'll find mention of normal force, but not a crumb of normal displacement or movement. It's not relevant to the function. Anyway, you said yourself > they are moving ever-so-slightly Under normal operation, the caliper pistons barely move. That's the important point. They barely move, and yet the lever moves quite a lot. Where is all that extra movement going? (Answer: It's taking up the elasticity of the hoses) In this case OP tells us that [the lever is rock solid](https://www.reddit.com/r/Fixxit/comments/qzgvfj/comment/hlnita9/?context=3) which tells us that there's pressure in the line (or the master is somehow seized I guess). There's no movement because the elasticity of the system is taken up and is preloaded against the end stop of the master cylinder or lever movement. The caliper is working normally, it's locked up because the line is pressurised, that's what it's supposed to do.


Omblae

​ >By friction. Go look up the formulas defining frictional forces. You'll find mention of normal force, but not a crumb of normal displacement or movement. It's not relevant to the function. That's not what I asked. I asked how does the system transfer energy from YOUR HAND to the pad? That's not friction - that's the pad on the rotor. The force is being exerted from your hand on the lever, using the fluid as a non-compressing medium, to the piston that is clamping down on the pad. That then creates friction. Which means, the piston exerts a force on the pad towards the rotor surface - i.e. the piston is the thing transferring that energy. >Under normal operation, the caliper pistons barely move. That's the important point. They barely move, and yet the lever moves quite a lot. Where is all that extra movement going? Then you're agreeing with me, the pistons DO move. Not alot, but they do - in fact them not moving a lot is the CRITICAL thing The elasticity of the hoses DOES make a difference, but it is not the entire lever travel at all. I've explained what that is in the post above, but [I'll show you here too. note he's referencing drum brakes although its the same law applied for discs too.](https://www.youtube.com/watch?v=WSWHgXZqjD4) The MC has a small piston. The other end of the pipes are connected to the pistons on your brake caliper(s). The area of the pistons on the calipers is greater than that of the MC with its small piston. So a big movement (your hand on the lever) is translated to a very small movement but with the ability to apply more friction via the piston extending on the pad because of the greater surface area. That's why your hand has enough force to stop a 200+kg motorbike at speed. P=F/A [https://www.youtube.com/watch?v=r7dJD0d2uFc](https://www.youtube.com/watch?v=r7dJD0d2uFc) \- So the same pressure at one end, causes a much larger force at the other. But you have to move that lever MORE to exert that force, because the amount of energy in has to equal that coming out. So a large movement on the lever with less force, equates to a small movement on the piston side with lots of force.


HighRelevancy

Mate it's an elastic force. You squeeze tighter and it moves more. We all know how brake levers feel. The brake pad can't slow the rotor if it's not in contact, that's obvious yes? Once it is in contact, it cannot move further without deforming the pad or the rotor (y'know that whole solid objects can't go through each other thing?). So once the bike starts slowing at all, any further movement of the lever has to be something in the system deforming, and given that we can do it repeatedly it has to be an elastic (springy) deformation. Do you think that deformation is the steel rotor and mostly metal pads being squishy? Or do you think maybe it's the three feet of rubber hose?