[epv]

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?

 

[epv]

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.

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.

 

[epv]

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.

 

[epv]

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!

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!

 

[epv]

Yeah it makes sense that crud could foul the seal in the ABS pump. I'm still not really grasping where fluid bypassing that seal would go though, other than to the calipers. Anyway, the bike has had all its services and fluids changed on time. I'm half tempted to bypass the ABS pump just as a diagnostic but I don't really have a place to tear the bike apart to get at it. Can't really ride it much now though as the master cyl doesn't seem to hold pressure for long.

 

[epv]

A damaged seal will allow fluid to bypass the piston so instead of forcing fluid down to the calipers, the fluid doesn't get displaced. The piston just moves thru the fluid in lieu of moving the fluid. Hmmm, guess it's hard to describe.

The fluid doesn't actually have to escape if the piston can't push it due to damaged seal.

Sure, and that's the working hypothesis, that the piston doesn't seal against the master cylinder wall, and pushes past the fluid. The only objection is that it seems weird to get two bad ones in a row. Still going to have the shop go over it again with the new m/c, it's worth spending another $100ish for some diagnostic work from people who actually know what they're doing. (hopefully)

 

[epv]

This is exactly what happened to me on my 2015. I put on a brand new master cylinder $600 but still wouldn't hold pressure. Before I bought the master cylinder I had a KTM dealer power bleed the system including the ABS unit but it didn't work and they (like me) believed the mastercyclinder was fcuked.
After I installed the mastercyclinder with no success I pulled apart the brake calipers and replaced all the seals again it didn't work. I really didn't think the seals were a problem as there was no fluid leaks anywhere but I thought I would eliminate that possibility.
I then did something that I should have done first up that would have saved me a lot of time and money, lesson learnt. I bought two long banjo bolts then uncoupled the brake lines from the ABS unit and joined the lines together using the longer bolts. After bleeding the brakes they became rock hard and stayed that way.
So it was the ABS unit all along but did not show up any faults and the KTM dealer didn't pick it either.
A new ABS unit here in Australia cost $3500 but there was none in the country. After putting down a $500 deposit I waited three months and it still hadn't left Europe because of the financial problems KTM are having. The dealer couldn't tell me when it might turn up so I got my deposit back. I now ride the bike without the ABS and traction control, that's not an issue for me but I guess it will be if I ever sell it.

Yeah, that is starting to seem plausible. I see ABS pumps for sale but they mostly seem to be used, out of stock, or from sketchy websites. I recall reading somewhere that even if you get one it's not as simple as just swapping out the faulty one, but I can't remember the details.

 

[epv]

I bought two long banjo bolts then uncoupled the brake lines from the ABS unit and joined the lines together using the longer bolts.

Curious about this - did you just disconnect the front master cylinder line MC1 and the line "F" going to the splitter for the front calipers and join them directly (like with a double banjo bolt & cap nut, or what? not a two-port manifold thingy?), and then cap off the holes in the ABS unit? Did you do the same for the MC2 / Rear lines? Did you have to do something for the brake lights to work?

 

[epv]

I disconnected the lines off the ABS unit and joined them together with the double banjo bolts, rear as well and plugged the holes in the ABS unit. You have to ignore the ABS not working light but the brakes work great. I have never owned a bike with ABS before so I can live without it, however I will eventually get a new one when I am ready to sell it.

Fair enough. I've had more bikes without ABS than with, so I don't much mind either, it's just not safe to ride without bypassing it because on sustained braking it gradually loses pressure. How did you assemble the banjo bolts - something like this?

Fittings - Banjo Fittings - BelMetric

belmetric.com

I suppose as long as it holds up and doesn't leak!

 

[epv]

I used these I bought on line. The washers have rubber inserts.
View attachment 122902

thanks very much for all the info! I’ll probably do the same thing to at least be able to ride it more safely.