So I noticed the other day that my front brake fluid was low -- below the bottom line. I went out and got some DOT 4 fluid to put some more in. As soon as I loosened the cap a little, the fluid level rose up to a point about where the top line was. I didn't touch the brake lever.

I just tightened the cap again and went for a ride. Front brake seemed fine.

Anybody know what caused this? Should I be concerned?

Thanks. -bg

If you didn't add any,and it came back to it's original level....all should be good.Did you change the front wheel or anything before noticing that?Maybe new pads or something?.New front tire switch?No leaks anywhere?How about pads....are they okay or really worn.Maybe the pistons are beginning to get dirty(if that's possible),and not going through the whole compress,release function all the way?

* Last updated by: blue07 on 5/24/2010 @ 11:32 PM *

Just a guess here--. Gas expands more than liquid from heat. Your brake system was warm from use or ambient temp. the air at the top of the reservoir was expanded and the whole contents of the brake system was under pressure. When you took off the cap, the pressure was released and the fluid came back to the correct level.

Same sort of thing happens with a gas tank. You hear the escape of air when you open an empty tank. Kinda fun to listen to if you ask me.

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How many miles on the pads? What percentage [take a guess] is left, if you looked at that slot in the pad; you can see through each disc side.

That diaphragm in the master, sits over that top level or filler line. You go over that fill line, she pushes fluid out the sides, down and out the master. Either way, that diaphragm has seal out the air. Now you see this diaphragm expand and drop down into the master. This is like an accordion dropping down, following the fluid. You release the cap, the fluid that was hugging the side of the diaphragm and master wall, drops back down to that level.

I've never heard fluid being pulled up the hose? You'd think you'd break the seal of the diaphragm > as soon as you disturb the cap. How could that pull fluid up, when you are pressing fluid down. Maybe the diaphragm has something to do with the dispersal of the fluid level is my guess?

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Ya...I thought that scenario was a bit odd as well.Never heard or seen it "come up the line".But I've had mine drop from pad replacement...which actually is kinda strange too(it's possible I'm getting "my" brake experience mixed up with my clutch levels experience!).Shouldn't it be the other way?I mean,if yer pushin the pistons "in" towards the calipers,isn't it gonna make the fluid go "up" the line???????Hey Hub...I don't think the diaphragm actually "pushes" the fluid "down",I think it only drops as the fluid drops so there won't be air floatin around in there.Isn't that how it works?I'm not sure if there's any real pressure in there or not...unless it was expanding from heat at the calipers and "filling" the reservoir back to "overfilled"?I guess it could then cool off sufficiently to suck the fluid back down,makin it look like it was low?IDK.

* Last updated by: blue07 on 5/25/2010 @ 12:33 PM *

blue: Front tire was replaced a month back, so the wheel was off for a while. No leaks. Pads seem OK.

Rook: Sounds like a good theory.

Hub: 14K some-odd miles on the bike. Will check out the brake pads tonight.

Thanks, guys... -bg

I'm getting "my" brake experience mixed up with my clutch levels experience!)

Here, look a it this way. Say you take a new #2 pencil, sharpen till you have a point a the lead end. Think of the yellow painted wood as all the fluid in the line and that is it. The eraser is the diaphragm and that will expand as you pull on it. So the eraser stays put, the pencil or fluid slides out of the band that is pressed around the eraser and pencil body. See the lead tip being the brake pad wearing down? The pencil begins to fall lower as the point is being written off as pad thickness thins down?
Notice the eraser acts as a vacuum only on the surface of the fluid? In that it is being sucked down and it stretches and follows the tip down as the diaphragm's accordion body opens. Remember now, there is 14.7 psi behind the diaphragm via the cap's vent hole. So, there is no vacuum at the diaphragm per say, to pull the fluid back up? How can it if there is no vacuum but a stable 14.7, no under vacuum or over pressure exists, if you look at that way. Just a piece of rubber being sucked down as the caliper piston moves farther out, then stops there as the pad wears. Yes, the Quad-ring pulls that piston back and whatever fluid moves back up the line is not much! If anything, the expanded rubber pressure is going to be pushed back up the master's return hole, before that q-ring could wrinkle how much out of its machined groove? Not much really.

Shouldn't it be the other way? I mean,if yer pushin the pistons "in" towards the calipers,isn't it gonna make the fluid go "up" the line???????

Count the clutch friction pads as one brake pad surface; If you count 9 or so plates stacked up as one. Does that look like a brake pad all built up as pad material... sans the plate as the carrier of the friction blocks bonded around the plate? So again, there goes the displaced fluid out of both master cylinders, down the brake line, behind the caliper piston.

As the piston stops against the pad, that Quad-ring still stays in place, but the piston move through that 0-ring hole. Once the push goes so far, the q-ring ever so slight, retracts backward. This and the fluid pressure, relaxes that hard rubber back to square. Can you now see the same fluid is going to drop down as the material vanishes in the air, or clutch fiber matter suspends in the oil, eventually lays on the bottom of the sump case; once it falls out of the oil?

Hey Hub...I don't think the diaphragm actually "pushes" the fluid "down",I think it only drops as the fluid drops so there won't be air floatin around in there.Isn't that how it works?

Yesir!

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I do know if you push the slave piston in after it's been allowed to move out so far,the fluid WILL rise back up into the reservoir.If it it continues to "move" outward,you will begin to lose fluid at the slave piston.I've had that happen.But I've never heard of fluid coming back up a line like that(except with the clutch line).It's odd anyway.If it expanded,it should have pushed the diaphragm back up into the cap where it was to start.I think there's something going on with Gordon's pistons.The pads aren't retracting far enough or something.No leaks,no air.Closed system.WHERE would the fluid disappear to?The volume is the same.The only thing maintaining the level is the pistons/pad thickness and the amount of fluid in the lines.

I do know if you push the slave piston in after it's been allowed to move out so far,the fluid WILL rise back up into the reservoir.If it it continues to "move" outward,you will begin to lose fluid at the slave piston. I've had that happen.

You do not lose fluid behind the piston. Then if you lost fluid, it is out the banjo, out the nipple, out the quad-ring. Show me the loss where you would have a spongy lever pushing at fluid with an air bubble inside. The tiny rise back is whatever expands in the line, whatever extends the piston at the caliper, whatever the master piston takes up then all is returned back. Remember that master's piston has to compress the fluid. So right there, you see how very little is going to return if the master is first loading up?

But I've never heard of fluid coming back up a line like that(except with the clutch line).It's odd anyway.If it expanded,it should have pushed the diaphragm back up into the cap where it was to start.

How can it push the diaphragm back up, when you already flatten the pencil point getting your point across. If you were to write this out, you'd be at that sharpener about 3 times already, someone change the pads, Hurry! Oh, yer gonna grasp this! Simmer down, Flow Chart, DIS is not rocket science [hand over mouth img].

I think there's something going on with Gordon's pistons.The pads aren't retracting far enough or something.No leaks,no air.Closed system.WHERE would the fluid disappear to?The volume is the same.The only thing maintaining the level is the pistons/pad thickness and the amount of fluid in the lines.

There is not a thing wrong with bg's brakes if he came back, said no change. After all those miles, one questions the fluid, the other questions where the fluid is going, the bike tells you that you are assuming something is funny, or something is wrong? Remember, this is the internet. One man's cap removal is another man's hand waving, yelling there is a problem with the pencil sharpener, someone point the way to another one!

You have this accordion effect of each fold sort of pops 0ut in place. Once that one accordion fold opens, another fold is to follow. That diaphragm is stuck on the suck, as it follows the amount of fluid the bike started with. The fluid that went nowhere without a leak to be told, but travels farther down the line; sits in back of the caliper's piston. There is your lost or can't figure out why the plastic fill line didn't follow the fluid. It displaces that cavity the piston came out of. The Quad will have no pressure on it via the piston can rock back and forth but eventually move out of the caliper, obviously and stay there. It's all sort of subtle are the moves.

No fluid is going to head back up, sitting in that piston's machined out chamber of the caliper it just filled. Speed the brake pad wear and how will the oil return> Did the diaphragm follow the flow? Did the master's piston push fluid against it's now closed holes in the master? All that happens now is that lever forming a closed hydraulic lock. You control how much pressure to apply on that brake disc.

You feel that modulation or warp. That is the disc coming around on the highest spot, pushes the piston(s) back in, your lever goes out. The disc is still turning, so you fell the lever go back to normal until that high spot comes around over and over. Did the piston at the master expose a hole it pushed it that far back? No. When you release the piston at the master, does not that need filling or neutralizing or back to no pressure at that the brake lines say?

Did not that one big hole in the master, have to fill that gap you reapply? Is not that tiny hole the diaphragm sucker? Kind of the [psi] neutralizer? Tiny hole sucks all that 14.7 back and forth, someone is going to eat pressure somewhere. Get it? Then we see how that diaphragm acts as that neutralizer and keeps that air from being sucked in the hole we get deeper down the pad say?

We sucking any of this up?

* Last updated by: Hub on 5/26/2010 @ 2:56 AM *

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Okay...the manual's full of shit....I'm full of shit.....I never had any clutch fluid escape out of the slave piston.And when I pushed the piston back in,the fluid in the line NEVER went back up into the reservoir.There...you're right,I'm wrong.Feel better? "that fluid went somewhere without a leak"....ya think?If she's pulling the diaphragm down,and she aint comin back up(diaphragm "returning" or not,the fluid(which is "original level") is NOT getting back to the reservoir.So if as I said,there's a problem with the pistons/pads thickness-the level is going to affectively "drop"even though it's the original amount...has to be...there's NO LEAKS.It can ONLY be the space around the piston/caliper fluid amount,or the pads are wearing(which is what I said in the beginning).That's it.Tell me I'm wrong.Rising back up into the reservoir as BG stated doesn't HAVE to be connected with him opening the cap.And it doesn't have to be pressurized to do that.If he removed the cap,and coincidentally the pistons returned back to their normal settings,the level would rise in the reservoir.I've had similar experiences and so have you...when I thought it was a cause of one thing,and really,it was unrelated,and "just happened" to occur at the same time or whatever.We all know that HUBSTER.Why then are piston parts(seals and such)replaceable.BECAUSE THEY WEAR OUT.And when they wear out.....things don't work like they're supposed to.Make any sense there Bro? "not a thing wrong with Gordon's brakes if he came back and said "no change"...no change from what?He said ...."I noticed".....not..."this just happened".It could have been like that for weeks.Is THAT too preposterous?I just last night "noticed" my clutch reservoir DIAPRAGM was being "pulled down".But actually,that's another assumption.It could have done that from something else.BOTH my reservoirs have displayed these things,and NOTHING was wrong.Mind you...IT WAS NOT TOUCHING THE FLUID INSIDE.Let me say here...for the record...these reservoir caps ARE NOT PERFECT.When I unscrewed the cap...there was no change in the fluid level.So what caused it?TEMPERATURE...that's what.You've got an empty airfilled void in the diaphragm.There's no fluid inside the rubber deal(granted,a tad,but nothing like in the res.Temps heat up,diaphragm air/void expands.Twice I've had to stop leakage(seeps)with teflon tape around the threads.If you READ the manual about the brake system/reservoir caps and such...it will tell you that they CAN STOP SEALING over time.Why else would they say "replace" such and such part "if".Sometimes,not even "if" but straight out "replace with a new one"after removing that certain item.Normally,I keep a good eye on these things.It WAS fine just the other day.Did it "drop" overnight?Do I have a leak somewhere...I searched...no leaks.So what's up with mine?I'm sayin...temperature has(as rook stated as well) something to do with these visual levels that occur periodically IF it's NOT something mechanical.Long as there's no leaks,I don't sweat it anymore.But I did not have to add any fluid either....it was the original amount in the reservoir...I know because when I last filled it,I did so to just above the low mark....and it hit me then"this looks kinda low",but I didn't add any cause when I put the cap on,it looked midpoint....which is fine for me.So HOW did the diaphragm MOVE then?'Splain that my man.Inquiring minds want to know. And STOP plagiarizing me! Nobody mentioned anything about the possibility that there's a problem with Gordon's master cylinder.At the check valve/return hole.It's there in the book.Parts aren't designed to last forever.I'd ask....any flakes or minute pieces of rubber at the bottom of the reservoir?It could be as simple as a fluid change...get any particles out of the system."No fluid is going to head back up".....so Gordon didn't actually see what he saw.It was a vision!OH LORD it's a miracle"...Gordon's seeing visions! And I didn't experience what I did either.And Jeffo can't tune a bike.Sound familiar? No Bro....we're not sucking any of this up.Who Loves ya baby!Just between me an you Hub Bro...yer gonna lose any credibility you have if ya keep sayin things like"It couldn't of happened" when someone says"this is what happened".I love this forum!!!!!WoHooooo! "Use the force Luke,use the force" What the heck...go ahead and rewrite my ideas....you know what people say...copying is the greatest form of flattery .

* Last updated by: blue07 on 5/26/2010 @ 4:23 PM *

So HOW did the diaphragm MOVE then?'Splain that my man.Inquiring minds want to know. And STOP plagiarizing me!

Stop? How about you quit assuming I'm going to copy your idea of how the brakes work, we are here arguing how a baby bottle would turn upside down and now show me how the milk leaves the vacuum as if that was the complete opposite you hold the liquid to the outside of that nipple, you want concept is to grasp the concept. It runs in the absolute and when we do not agree, someone is not up to snuff on their basic physics like the 3 amigos is count them off. Don't make a mistake! YOu either know this part at the nipple or you do not.

Not plagiarize, but 'place your eyes.' Place your eyes over the very first question and cover your ass or I will chew it up, you screw up the tech. BG is not coming back here screaming about bad brakes ANYTHING!

If you knew about that diaphragm; look, see if there is anything said in that book about reforming it back in the pre-load fold.

I'm going to take you for a LAP like 1bad is 2BAD I choose my threads carefully is because I know my shit till the cows come home you come mess wit me is mess you the yup)Pee on your due pee come step it UP is PAZ YOU THE FUCK UPPPPPPPPPPPPPPPee Dup Pee.

http://www.youtube.com/watch?v=bAZ0aaqTJjM

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Oh nevermind .

Wow, passionate argument about a non-ABS braking system! Only in a motorcycle forum.

No I'm wrong, the guys and girls in the Carrera forum get down worse about technical stuff sometimes. Or used to haven't been there in ages.

As far as I know, the only way hydraulic fluid (brake fluid) would leak out past the slave caliper pistons during a wheel change is if the seal behind it was bad. The hydraulic fluid doesn't directly work against the caliper pistons.

But I could be wrong.

As for the level changing in the resevoir the way BG describes, was the bike hot from being run or cold in the morning before running. The hydraulic fluid is not very compressible by definition, and hence not very expandable. But the metal components expand and contact some based on operating heat.

BG, if you don't have a soft brake lever, and it won't go all the way to the grip, and stays that way when you head the brakes up good, and you don't get fluid at any banjo joints or whatever, you should be ok. Air in the system wouldn't cause that to happen, if it did, the brake lever would go totaly soft.

But like I said, I only had my front brake lines totally off and replaced, and only needed one small bottle of hyrdraulic fluid to recharge the front braking system, and thats all the experience I actually have.

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Sorry Hub...I forgot oh master that thou wouldest smite me with thy furious vengeance were I to have a differing opinion or experience.Your meek and humble serf doth apologize in the most prostrate and reviled way.If only thou couldest find it in thine heart a small vestige of mercy upon thy foul servant?And forgive the blasphemy of mine dark heart.Be merciful oh high and lifted up one,for thy heretical dolt knoweth not the deep mysteries of the reservoir bladder.

* Last updated by: blue07 on 5/27/2010 @ 6:17 AM *

Sorry I haven't gotten out to the garage and taken a look at the brake pads yet. I would imagine that they are in fine shape. Actually, I don't use the brakes all that much/hard on a normal twisty canyon ride (which are most of the rides I go on). The engine usually slows the bike down enough for turns, unless I've just passed someone before a turn and need to scrub off some extra speed. That track day at Miller a month ago was an exception, however ...

privateer: The brake lever is not at all soft. Matter of fact, it's very hard since the fluid came back up. Pull it to a stop and you've barely pulled it in. Much harder than before.

Brake seems to work fine, as it always has. Just confused about what the fluid did...

Thanks for all the input. -bg

* Last updated by: bgordon on 5/27/2010 @ 8:28 AM *

Glad ya had a blast at the track Gordon!Sounds like yer fluid deal may be associated with your track performance?IDK...but glad she's working good.No leaks....excellent.Better than before...Perfecto!My opinion....nothing to worry about.

* Last updated by: blue07 on 5/27/2010 @ 8:41 AM *

privateer: The brake lever is not at all soft. Matter of fact, it's very hard since the fluid came back up. Pull it to a stop and you've barely pulled it in. Much harder than before.

Oh, you mean pinch a burp at the old master:

1. The BG Burp = Whatever the B did, you do it real slow to gain a trick lever.
2. The bleeder at the master = Hard pedal.
3. The 'HubHasIt' = Burp [push] the calipers [in] and there are your 3 burpers for that specially designed master cylinder.

Matter of fact, you learn something everyday. And how come you didn't catch those 3 variables I have now locked in the who cares file.
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And as simple as that, I've collected 3 tricks to lever the pull. Pull you around like a puppy dog is dog ear this page. 3 steps ahead I am finding fact after fact and private reserve has to see some concept as in that hot dog balloon. Fill it with water. Look like a brake line with dot4 all sealed no leaks?

Take a big hand on one end to hold it there. Little kid's hand has the other end. Big is the caliper. Small say is the master's piston vs. piston size is take a side? Now who has what pressure is equal. First come back, dog walk a step for step, there is equal pressure at one end is equal for every action there is _______________ Fill in the blank.

Oh look! Someone left a piece of pea saw wit cheezeIs can I get a yes or no or fill in the blankit statement is one word even!

* Last updated by: Hub on 5/27/2010 @ 10:19 AM *

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