Motorcycle Brakes Squishy Overnight

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Nov 14, 2013
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I recently purchased a 2017 Zero FXS electric motorcycle, it's been a blast for commuting so far.

Riding it home though, I noticed the brakes were a little squishy and the fluid in the reservoir was dark, so one of the first things I did was flush and bleed the front and rear brakes. I definitely purged the air and the lever felt nice and firm after the flush. However, the next day, I went to ride it and it was squishy again. As I pulled the lever, I could see air bubbles coming out of the master cylinder into the reservoir. As I flicked the lever, more bubbles came out and the lever got firmer. Thinking I left a bleeder loose, I tightened the bleeders (at the caliper and MC) and tried checking the banjo bolts, which all seemed tight. Next day, same thing, squishy lever.

After some googling, I found the suggestion to use a bungee or zip tie to hold the lever down over night to let the air out. I did that and in the morning, the lever felt firm again. I've been doing that every night just to get by, but I swear, within my ~30-45 minute commute, I can feel the lever get squishier.

FWIW, the caliper and MC are J.Juan brand, and it does have a Bosch ABS system. I didn't check the banjo bolt connections to the ABS system yet as that goes into the chassis. Also, I haven't noticed the fluid level in the reservoir change in the past 2 weeks since I flushed it, so I don't *think* there's a leak.

Any suggestions on what to check next? Since holding the lever in seems to keep air out, I'm inclined to think it may just be the seals in the MC and I need to rebuild it.

Thanks for any input!
 
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If your absolutely sure you have no leaks anywhere in the system try this. When you put the bike away for the night, use a clamp or a ty wrap and pull the brake lever as far back to the bars as you can, then sinch down on the clamp or ty wrap. Let it sit over night like that. Some bikes are hard to get all the air out of the brake system. If that doesn't work to correct the problem, you allowing air to enter the system somewhere.,,
 
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I had a situation where I had air getting into a braking system on a motorcycle that was still under warranty. Eventually got so bad that the issue was escalated to the manufacturer's team that had designed the braking system, for advice/guidance.

After replacing just about every component in the braking system (and being wthout the motorcycle for several weeks), they finally discovered the culprit. A bleeder with a crack in it.
 

EdwardC

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Thanks guys! I think I'll try replace the bleeders first, that's pretty easy.

I'm having a tough time finding the right rebuild kit for my MC, I've contacted the manufacturer for info. If I can't figure it out, I'll probably grab another MC to test.


BigCahuna, I've tried holding the lever down over night and the lever is nice and firm in the morning, but is squishy again after a few hours / overnight. I'm pretty sure air is getting in somewhere, just not sure where.
 
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Jul 11, 2021
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Frustrating for sure, I’m having a similar issue with a KTM from brake.

In the past I’ve resolved with by rebuilding the MC, always my first steps after all the checks you have already done.

Something to keep in mind. Sometimes a brake system will hold pressure from the inside but let air in from the outside somehow. Makes trouble shooting very difficult.

I would rebuild the MC(or get a different one in your case) for starters. Then the brake line(s) next. Then look at the caliper(s) next. Hopefully it’s not something with the ABS system, I know nothing about them as I steer clear of ABS on bikes.
 
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Aug 10, 2017
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South Oz, Australia
I recently purchased a 2017 Zero FXS electric motorcycle, it's been a blast for commuting so far.

Riding it home though, I noticed the brakes were a little squishy and the fluid in the reservoir was dark, so one of the first things I did was flush and bleed the front and rear brakes. I definitely purged the air and the lever felt nice and firm after the flush. However, the next day, I went to ride it and it was squishy again. As I pulled the lever, I could see air bubbles coming out of the master cylinder into the reservoir. As I flicked the lever, more bubbles came out and the lever got firmer. Thinking I left a bleeder loose, I tightened the bleeders (at the caliper and MC) and tried checking the banjo bolts, which all seemed tight. Next day, same thing, squishy lever.

After some googling, I found the suggestion to use a bungee or zip tie to hold the lever down over night to let the air out. I did that and in the morning, the lever felt firm again. I've been doing that every night just to get by, but I swear, within my ~30-45 minute commute, I can feel the lever get squishier.

FWIW, the caliper and MC are J.Juan brand, and it does have a Bosch ABS system. I didn't check the banjo bolt connections to the ABS system yet as that goes into the chassis. Also, I haven't noticed the fluid level in the reservoir change in the past 2 weeks since I flushed it, so I don't *think* there's a leak.

Any suggestions on what to check next? Since holding the lever in seems to keep air out, I'm inclined to think it may just be the seals in the MC and I need to rebuild it.

Thanks for any input!
My .02c... Seal kit for the master cylinder is required...
Maybe the new brake fluid reacted with the old and played havoc with the seals :unsure: mmm on that note change out the calliper seals while your at it, not much $ there. But I'd try the master kit first..
Primarily my rationale for this is that you had full pressure and you lost it over the trip, ergo you had all the air out until you started using it over a certain time, this is the basis of my analysis and subsequent recommendation.
(y)
 

EdwardC

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Thanks! Finding the parts fiche for the Zero was harder than other bikes I've owned, but I finally found the info for the brakes. Looks like the JJuan 12.7mm bore brakes, and even found the part number for the rebuild kit, which isn't too terrible at $30. I have that ordered, but they're special order parts so they're a couple weeks away.

I've rebuilt calipers before, but never a master cylinder, so I have time to YouTube it a bit.

I ordered a cheap aftermarket MC to test out tomorrow.

I didn't Velcro the brake lever last night, and it was very squishy this morning, you can see the bubbles coming up on this picture as I pulled and flicked the lever.

I, too, am hoping it isn't an issue with the ABS.

Thanks everyone!
 

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EdwardC

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Small update, I put in the aftermarket master cylinder, seems to have the same issue. I have the front caliper rebuild kit on the way already, so that'll be my next step. The MC rebuild kit is backordered, but I am thinking that's not the issue anymore. I had ordered a replacement bleeder with the MC rebuild kit, so that's not here yet, but I think I might just order one separately just in case.

After that, I might have to dig into the ABS a little more, or possibly just bypass the ABS.
 
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To eliminate the calipers and ABS module, just pinch off the lines shortly after the master cylinder, with something suitable of course that will not damage the lines. Wait overnight and see if the lever is firm or not. That should tell you if your MC is to blame.

Then move down the line and pinch off the brake caliper, leaving the abs module and see if it gets spongy then.
 

ZeeOSix

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If there is no external fluid leakage anywhere, then it's got to be leaking past an internal component. Pinching the brake lines would only take a leaking component (like a hosr fitting or brake cylinder) beyond the master cylinder out of the equation. But if there's no sign of any leakage then it won't tealky tell you anything by doing a line pinch test.
 
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What type of brake fluid are you using? If a previous owner installed Dot 5, that can cause major problems with ABS brake systems.
 

EdwardC

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I'm using Valvoline Dot 4. Can't say what was in there before, but my new fluid poured and mixed into the existing fluid in the reservoir with no issue.

I had always assumed that silicone and glycol fluids wouldn't mix, like oil and water, but I don't actually know.

Probably wouldn't hurt to flush more fluid through the system. I'm wondering if maybe I didn't get enough new fluid to flush through the entire ABS loop.
 
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My observation is of how factory bled KTM front brakes do not go soft overnight but once a dealer or myself has serviced the brake fluid the front brake will be soft every weekend when ridden. The soft brake can be quickly "bled" by spreading the brake pads with a flat blade screwdriver then pumping the lever to put the pads back in contact with the rotor. Clearly the air bubble is somewhere near the master cylinder.

In the years since my theory of the expansion bladder in the master cylinder reservoir having an air bubble underneath seems to be proven true. When ridden this air gets shaken into the brake fluid. This fluid migrates into the brake lines with use. When it sits the small dissolved bubbles form a large bubble. This flys in the face of my understanding of fluid dynamics and air mixed with fluids but I don't know of any other explanation. Small dissolved bubbles should be as compressible as one big bubble. That when ridden the brake is firm, overnight the brake goes soft. Doing what I can to minimize the air caught in the folds of the bladder in the top of the reservoir seems to minimize the problem.
 

ELS

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I'll second that. I had this issue with a race machine. Sometimes it could be so bad that first grab of the lever was no brakes.

Interestingly, I started pre-race checking it, and started removing the reservoir cover and bladder. Pump a couple of times and they were firm. But then somewhere in the race I would reach for brakes and have to pump to get any. For a bit I thought well it must be the master cylinder after doing a complete fluid change out by bleeding.

In the end, I think it was the dang master cylinder cover from one of my sponsors. It could have been to do with the bladder or how it interfaced with it. All I know is that I put the stocker back on and the problem was gone.
 
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