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Discussion Starter · #1 ·
I "think" I've solved the notorious rear brake issue with the my15-18 monsters. After finally getting fed up with bleeding it for hours, getting a good feel and then it immediately fading as soon as I went for a test ride I decided enough was enough. I installed a brand new rear master cylinder and g34 brembo caliper. I have a feeling the seals on the mc start to wear at around 10k miles or so, since that is when I started to notice it, I'd like to hear from anyone else who has had this problem if this is the approximate mileage they had when it started to crop up. The feel when bleeding between the old and new mc is like night and day, you can actually feel the mc refill from the reservoir in-between bleeding unlike the old one. I only replaced the caliper because I wanted to be 100% certain there were no issues there, and I changed it to titanium color so it matches the front.

The cost for a rebuild kit for the brembo rear mc is between $20-$30 when you can find it. I picked up the brand new MC for ~$60, and I didn't have to deal with the hassle of tearing down the old mc.

I'd reccomend to anyone who has been fighting this to try replacing the MC. It may just be that one of the inner gaskets wears down after x miles and that causes the slow air leak when sitting or the outright mush when using it.

I will update again in a week or so if anything changes, but consider no news good news.
 

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Give it 2 months....report back....stand on your brake pedal now with a ruler behind it...photo...I just bled mine...super tight...will do the same photo and post it...mine still drifts but I don't believe the master is at fault...the abs unit on mine was definitely full of air early on...but I'm able to pull air in just by not pressing the pedal fast enough while bleeding...we need evidence to say it's solved...take the photo...be fair... we'll see
 

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Discussion Starter · #3 ·
Give it 2 months....report back....stand on your brake pedal now with a ruler behind it...photo...I just bled mine...super tight...will do the same photo and post it...mine still drifts but I don't believe the master is at fault...the abs unit on mine was definitely full of air early on...but I'm able to pull air in just by not pressing the pedal fast enough while bleeding...we need evidence to say it's solved...take the photo...be fair... we'll see
I've used a vacuum bleeder, reverse bleed, flipped the caliper, bleeder ontop of the master, etc and nothing solved it. This is the only thing I've done that even produces a rear brake grip for more than the 5 minutes following.

With the new MC, moving the pedal by hand you can feel when the line is at full pressure and and then if you put all your weight on it you can feel that it is has more to go and is not bottomed out like when it was bled before. Even when bleeding the difference is quite clear, as once the air has been pulled with the vacuum, you get a good solid consistent pull of fluid sans bubbles.

I was at my wits end and finally gave in to try this and so far its been a complete improvement, and I think if like me, you got tired of spending hours trying to bleed the line, $60 is not a bad investment. I will provide updates as I continue riding.
 

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Odd, your monster is opposite to my SS.

I’ve bled it to get the pedal.. but after it sits the pedal goes away.
I’ve long given up and just ride out the drive.. I keep using it as normal and 10 - 15 minutes riding it is usually back up to normal.. sometimes 20-30.. good to go for the whole ride or until next time
 

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I'll post a video on my technique this week...I find if you don't lean into the pedal at a certain cadence it goes soft even if you had already purged it of air... something is not quite right in the system... perhaps with a bit of maths...someone could calculate the distance pumped with the arrival of air bubble at the nipple...x# = master cylinder or x# = the distance to a crimp in the brake line...but I can induce it simply by not pressing the pedal hard and fast enough...wether it's a master cylinder rebuild or a new line...or even a temp thing...no more guessing...let's get the datum...

I'm happy that your brake is working but I'm clearly skeptical of the longevity

My brake was functioning at this last bleed...the pedal travel was double of what it is now but the brake did work....and it lasted longer than it ever had...(motul 600f)...no rebuild; no new line; no insulation...I'm not convinced it's a solution...pedal still goes weak over time...hence the ruler...measure the travel now and again in two months....the culprit will eventually be exposed...

On another note....anyone reading this that has replaced any brake component...has your pedal travel been sustained over time?
 

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Are you bleeding manually or with a vac?
I had a vac...I broke it in a binge bleeding session...lol...never worked fast enough for me... manually stomping the pedal was the quickest way to move fluid through the line...i've wanted to get another one...just couldn't bring myself to buy it
 

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Discussion Starter · #8 ·
I had a vac...I broke it in a binge bleeding session...lol...never worked fast enough for me... manually stomping the pedal was the quickest way to move fluid through the line...i've wanted to get another one...just couldn't bring myself to buy it
Might want to try another vac because that might be your issue. You aren't trying to move the fluid through the line, you are trying to force the air out. Pumping the pedal forces the air to compress as the fluid cannot, which is usually at the highest point (why you flip the caliper to out the bleeder above the system). The vacuum bleeder creates a low pressure vacuum that draws the air as it is more easily compressed and pulled into the vacuum (like with pumping) than the fluid.

If you are trying to bleed the system by just moving fluid through it then all the air will still remain trapped.
 

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My issue was the fluid wasn't moving fast enough with the pump...so air was remaining...it never got tight even with 1/2 liter...by using the pedal I can cycle two reservoirs worth and be done....and air gets wooshed along with the fluid...the vac definitely made keeping the reservoir full easier...three or four good pumps and it's empty using the pedal...I'll hold the caliper with two hands and lean into the pedal with my knee...rocking back and forth it goes really quick
 

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Discussion Starter · #10 ·
You shouldn't be pulling that much fluid if the brake system is working correctly, the bleeder only needs to crack a tiny amount. Pumping the pedal before opening the bleeder compresses the air to the highest point, and opening the bleeder allows the pressure behind from the fluid to force the air out.

The vacuum creates the negative pressure to draw the air with the fluid out, which is why using it in conjunction with pumping is so effective.

If you are having to pump the pedal so much to move all that fluid through to try to capture the air with it, then something is wrong in the system and is allowing air into the line.
 

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I am quite certain air is getting in somewhere...that was kind of my point...im pretty sure I can induce the air by doing nothing more than pushing the pedal a bit slower....its nearly immediately felt in the pedal....it goes from firm to sponge....then I crank it a bit more and a cascade of tiny bubbles come out the bleed screw....my hope was to calculate the amount of fluid passed which passed through the line to determine the origin....

When my pump was operational; I was cranking the vacuum up to near the top of the gauge...was squeezing so fast it had to look like I was a cartoon character... ultimately did it in...handle snapped...but even with the constant draw the pedal never got solid...of course this was a long time ago...I've always wanted to get another...but I need a secondary reason other than brakes... and it hasn't come up...
 

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Forget the ruler...it's proving difficult to get an accurate image....but how's this look to you...on my bike this is about as good as it gets...

...and tracking the air bubble was tricky...and to be fair it would need a consistent draw of say a vacuum pump to really have any validity....I pump... I fill ...who knows how far the bubble drifts and at 24.00 a pint my experiments need a bit more precision to approve the cost...lol...but still...the pedal goes limp almost immediately if I bugger up the timing of the bleed screw and the air takes nearly 1.5 reservoirs to pass...the pedal will get the pads to bite but you can see it seems a bit soft...I definitely would be able to modulate it....so normal? It's hard to judge parked but I've been on others were it seemed to grab almost immediately

 

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Discussion Starter · #14 ·
Yeah it shouldn't be spongy like that after bleeding. I'll try to take a video of mine when i get the chance, but there should be a clear wall when you compress the pedal where the brake is fully biting, without the pedal bottomed out.

If I pump my pedal all the way up and then crack the bleeder, it will move another 1/8 to a 1/4 inch because the fluid has an outlet. If the pedal is bottoming out after bleeding, something is wrong, and my guess would be the MC.
 

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can’t see it being the master.. they are a simple single line cylinder. If it is bypassing it will go all the way.. not just a little bit.
If it was letting air in the fluid would drip into boot and onto the floor.
It’s not unusual to have a little ‘squish’ but not much.

here are some things to think about. It looks like you have some flex.. but more likely there is some run out in the rear rotor / wheel / bearings / or pad knock back. Calliper pistons and stick and pull too far back into the calipers.
Pads themselves can flex and pads can stick in the caliper carrier thereby flexing.
If your getting lots of bubbles in your bleeder tube then I reckon you’re opening the bleeder too far.. get some speed bleeders from motowheels.
Your master cylinder looks like it mounts vertically.. guess where any air sits. Remove the master and caliper.. from mounts

personally i gravity bleed most of the time.. crack the banjo bolt, about 30 secs.. then nip it, top up fluid, spin the caliper and remove the bleeder.. get it lower with the bleeder up.. let it flow((“away from pads). Top up reservoir

lots to think about

oh one more. You are not the bloke with Parkinson’s that can’t help shaking the brake fluid bottle before opening it are you?
 

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I am half tempted to to hang the bike from the rear tire...lol

I was given a loaner m821 when my bike was in the shop long long ago...they never fixed my problems...but the rear brake on the loaner...a demo....I thought it was broken...it had nearly no travel and worked...rather than say something to them about mine...they haven't sorted anything...I took this to be the target...it just recently started to hold pressure...before it would go full stop in short order and no holding pressure...the higher temp brake fluid made a difference...the thing worked...but in the video you can see it's not particularly solid...

And I'm aware of spring back...the pads are held apart with a t-allen key off the bike inverted...and any air entered in the bleed screw should be seen immediately or shortly after bleeding resumes...but that's no the case...it's nearly 1.5 reservoirs before the little bubbles pop out....

I've been reluctant to spend any money the issue because I just rather use the engine breaking...and the resolutions are basically the entire system...multis went with masters and insulation...monsters going with lines...my initial failure was abs...the master at this point is out of convenience...it's a pretty simple swap...just had the bike apart...it's surgery for the line...if this isn't it then that's next up

And in 67k miles I haven't so much seen an accumulation of grime anywhere...but it's creeping in somewhere...maybe the banjo bolt? Something up high?...maybe taking it off and back on will do it?...I miss the days of hitting things with a hammer...lol

Brake fluid has been at rest for weeks...and I don't put exiting good fluid back in...I can set up a go pro to film the entire process...need to charge it
 

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Discussion Starter · #18 ·
It would depend where the wear or leak in the MC is. If it is at the reservoir elbow, then when the pedal is depressed it wouldn't force fluid out but when it is released it could draw air in. It all depends on which seal is the would be culprit.

If the line is being bled and no matter how much it is being bled there is still air in the line, then there is a failure somewhere above it just being a poor design.
 

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I'm thinking about it and yeah I've got the same amount of flex in the front brake I'm going to push the pistons into the caliper all the way and see if I get any new air bubbles... we'll see
 

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It would depend where the wear or leak in the MC is. If it is at the reservoir elbow, then when the pedal is depressed it wouldn't force fluid out but when it is released it could draw air in. It all depends on which seal is the would be culprit.

If the line is being bled and no matter how much it is being bled there is still air in the line, then there is a failure somewhere above it just being a poor design.
Curious if a dab of grease at the elbow is all we need? Something else to try...
 
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