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Front brake master cylinder rebuild?

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#22 ·
Never a good idea to revive a dead thread. Best to start your own thread with all relevant details for best success. Your solution is to reverse bleed air out of master by compressing caliper pistons a bit to force air back out. This will work temporarily. Rebuilding master cylinder can help for awhile but new replacement master cylinder is the permanent fix
 
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#24 ·
A friend (who is a master mechanic and engine builder) had to bleed his Aprilia Tuono V2 Factory front master once a month to maintain braking pressure. Never did figure out why in several years of ownership. Evidently, some manufacturing defect causes some Brembo mc's to suck air. Possibly the splitter block under the bottom triple clamp has a porosity issue?
 
#25 ·
I have a theory. I suspect warm brake fluid cools and shrinks fluid displacement and draws air into master cylinder as fluid cools. Just a theory
 
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#26 ·
According to this, your theory is plausible. If the ventilation of the reservoir is blocked or restricted, the4 resul;ting vacuum wi9ll tend to pull air in from wherever it can. Racebikes use socks around the res to soak up spillage, that, without ventilation, could not happen.

bgfindashop.com

Do I need to change my brake fluid? - BG Find A Shop

As vehicle owners, we assume our vehicles will be reliable. We trust that the brakes will stop the car, every time. Most people don’t really question how the
bgfindashop.com bgfindashop.com
.
 
#27 ·
Hammerhead said:
According to this, your theory is plausible. If the ventilation of the reservoir is blocked or restricted, the4 resul;ting vacuum wi9ll tend to pull air in from wherever it can. Racebikes use socks around the res to soak up spillage, that, without ventilation, could not happen.

bgfindashop.com

Do I need to change my brake fluid? - BG Find A Shop

As vehicle owners, we assume our vehicles will be reliable. We trust that the brakes will stop the car, every time. Most people don’t really question how the
bgfindashop.com bgfindashop.com
.
Click to expand...
Didn't know that was the reason race bikes used socks. I thought it was just to cover up the clear reservoir. Sounds like I'll end up doing a bleed. It's just weird that for the first year and a half there was no issue and now out of nowhere just sitting in the garage there's no pressure. It usually just takes one crack on the bleeder to get pressure back up.
 
#29 · (Edited)
In the garage I can pull a soggy lever enough to hit the cluster but in use there haven't been any issues. When the brakes are well bled I have to pull hard to get the lever touch the cluster. There is enough adjustment in the lever to adjust any issues away and less lever travel is needed so it works out. Again, the RCS is awesome. Even with a soggy lever there haven't been any issues.
 
#31 ·
This **** is driving me crazy with my '14 SDR. Lever goes right to the bar, but if you pump it a couple of times it's completely fine. Then let it sit a few minutes it's back down to the grip again. I've replaced the entire master cylinder with a brand new $400 OEM one, flushed out the lines, tapped all the fittings and junctions, no bubbles at all from anywhere when bled either at calipers or MC.

Sucks when you're riding, because you never know how much pressure you have to give to get a given amount of braking force - it's super sketchy and feels dangerous. I can feel the lever sinking toward the bar while braking down a hill, for instance.

On the one hand, there's absolute zero leakage of brake fluid anywhere, so how can it be anything but fluid bypassing the piston in the master cylinder? There's nowhere else for the fluid to go, and the level in the reservoir never goes down. But on the other hand, it's a brand new OEM M/C straight from KTM, so how is it gonna be failing like that?
 
#32 ·
More than likely loose caliper mounting or possible brake hose failure. Hope you checked master cylinder lever free play. Too tight or zero clearance will also cause your symptoms.
 
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#33 ·
augiedoggie said:
More than likely loose caliper mounting or possible brake hose failure. Hope you checked master cylinder lever free play. Too tight or zero clearance will also cause your symptoms.
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Everything with the M/C seems reasonable and when it's not being spongy it's exactly expected. It's just sometimes it decides to sink right down to the bar. I'm curious what you mean by caliper mounting - do you mean the caliper being oriented so that weight or stress tends to push the pistons back into the pots? I guess this is a possibility worth looking into.
 
#34 ·
Be sure to adjust master cylinder lever free play exactly as specified in your service manual. This setting is critical to brake operation. Your master cylinder converts your lever pressure to hydraulic pressure and it's transmitted to calipers where it's hydraulic pressure is converted to mechanical pressure again. Your caliper mountings or rotor mountings or anything mechanical can cause excessive travel/movement to dissipate more hydraulic pressure/stroke. Imagine your master having the ability to generate 1 inch of movement to fully lock calipers normally but wear or damage has now required 2 inches of movement. Your lever needs to travel further. Loose wheel bearings, loose caliper mountings can all cause this additional travel. Lift front wheel off ground. Watch caliper movement when applying brake and rotating front wheel. Observe any excessive movement or travel.
 
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#35 ·
I had a similar issue with my BMW S1000's rear brake. Replaced MC but didn't sort the issue. Took it to dealer to get bled using BMW computer where it was diagnosed as a bad ABS pump. At ~$2k, told dealer no way. Bought new rear brake line and bypassed the abs pump which worked fine.

You can try KTM shop for bleed but will likely have same issue. Maybe it's the ABS pump and you want to retain ABS so you buy new pump. But you also have an option to delete ABS with new brake lines directly from MC to calipers for a fraction of the price.

Good luck.
 
#36 ·
Just as an experiment I loosened the caliper mounts to avoid any pressure that would tend to push the pads apart. Zero movement of the calipers when the brakes are applied, which is disappointing since it's one of the few other plausible explanations, heh. Lever free play is adequate, there's zero pressure on the piston until there's a bit of lever travel, and when there's sufficient pressure, the full travel of the lever ends well short of the bar.

It's just that after no lever squeeze for a minute or two, the first pull is very soft. The second pull is firm and behaves exactly as it should. It's almost as if the pads got slightly pushed apart in between, but I'm confident they're not.

The shop seemed pretty convinced it's a master cylinder problem, but.. this is a brand new one, new brake fluid, bled at the calipers and the master, etc.
 
#37 ·
epv said:
Just as an experiment I loosened the caliper mounts to avoid any pressure that would tend to push the pads apart. Zero movement of the calipers when the brakes are applied, which is disappointing since it's one of the few other plausible explanations, heh. Lever free play is adequate, there's zero pressure on the piston until there's a bit of lever travel, and when there's sufficient pressure, the full travel of the lever ends well short of the bar.

It's just that after no lever squeeze for a minute or two, the first pull is very soft. The second pull is firm and behaves exactly as it should. It's almost as if the pads got slightly pushed apart in between, but I'm confident they're not.

The shop seemed pretty convinced it's a master cylinder problem, but.. this is a brand new one, new brake fluid, bled at the calipers and the master, etc.
Click to expand...
Unfortunately the last piece of the puzzle is the ABS pump. If it's piston/seal is leaking you'd never know it externally as the fluid would just bypass it.

Hopefully a good bleed with the dealer tool sorts it for you but go in prepared for the worst.

Good luck!
 
#38 ·
Lowrance said:
Unfortunately the last piece of the puzzle is the ABS pump. If it's piston/seal is leaking you'd never know it externally as the fluid would just bypass it.

Hopefully a good bleed with the dealer tool sorts it for you but go in prepared for the worst.

Good luck!
Click to expand...
That's what I originally thought it was, and I'm still not convinced it isn't, but I took it to the KTM dealer and had them run the diagnostics on the ABS control module and pump and bleed everything, and they said it was good and recommended replacing the master cyl.

ABS or not, if fluid is getting pumped down the brake line from the master,i feel like there's not really anywhere else for it to fill into except the slave cylinders, and they're already full and clamped onto the discs, and there aren't any leaks that i can find. Pumping the lever repressurizes everything and there's no fluid being lost at all.

Regardless, it's really helpful hearing input from people here because I feel pretty clueless!
 
#39 ·
The front brake line goes from the MC to the ABS pump, another line runs from the pump to the splitter under the lower triple clamp and then down to the two calipers. If there is any kind of foreign material or debris in the brake fluid it can foul the ABS pump seal.

What year is your bike? Has the brake fluid been flushed at interval spec'd in manual? If not, debris can foul up the pump. That was the issue I had with my 2014 S1000. As I mentioned, I bypassed the ABS pump as I was not willing to spend $2k for a new one. Bike was less than a month out of a 3 year warranty which just added salt to the wound.

Anyway, hope you get it sorted.
 
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