Front Brake Spongy - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 18 (permalink) Old 04-06-2019, 02:18 PM Thread Starter
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Front Brake Spongy

Before I begin, my front brake was working well last season.

Over the winter, I flushed the brake fluid, replaced the front tire, and replaced the rubber boot that goes over the bolt coming out of the reservoir.

When bleeding, the brake never firmed up. I put a clamp on it overnight, and it firmed up. But after sitting for a few days (I haven't ridden yet), I can easily pull it all the way back to the handle. If I clamp it back overnight, its firm again in the morning. But a few days later, it's back to being able to pull it back to the handle with ease.

There's no sign of leaks. Fluid level is halfway between the upper and lower marks. Any ideas?

It will be a bit before I can ride it, as I resolve unrelated issues. But I am concerned about being able to stop.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #2 of 18 (permalink) Old 04-06-2019, 10:01 PM
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Yeah, brakes can be a pain. The good news is that they usually resolve themselves.

There's a couple of different things that you can try. One bike I had in the past had a hydraulic clutch that was very difficult to bleed. There was a lot of chatter on the forum for that bike about different strategies. The method that seemed to work best was to cover the gas tank as brake fluid will do a number on your paint job. Then apply some pressure to the lever and loosen the banjo bolt at the master cylinder. Force a small amount of fluid out (and hopefully a few trapped air bubbles), then re-tighten immediately before releasing the lever or you'll be right back where you started. So, that's one way. . . . Won't do much good if the air bubbles are trapped down at the caliper, though.

I upgraded the 454 that I bought for my son a couple of years back to stainless brake lines when I bought it. The symptoms were very similar to what you're describing. I could pump it a few times and get good pressure, then come back the next day and same problem. With that bike I solved the problem quite by accident when I changed out the front tire. I hung the calipers with a short length of cord from the frame while I was working on it. Never had a problem after I put it all back together. I figure the calipers were probably inverted, and the bubbles accidentally found their way up the brake line and escaped by themselves while I was putting everything back together.

So maybe you could try one strategy or the other and see if you have any luck. Otherwise you could try bleeding the banjo bolt at the caliper, too. I'm guessing you're not getting any bubbles when you bleed it using the bleeder at the caliper, right?

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post #3 of 18 (permalink) Old 04-14-2019, 07:37 PM Thread Starter
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I bled and bled and bled, but the lever never got firm, despite no bubbles. Clamping it to the bar was what finally got it firm.

But it goes bad in a day. I clamp it overnight and it's good again for a day. My reservoir level seems to be going down, but I don't see leaks anywhere. It HAS to be going somewhere. Air can't just reenter the system without evidence.

I don't understand what's going on.

I used DOT4 as the 2002 service manual stated, but my reservoir says DOT3 on it.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #4 of 18 (permalink) Old 04-19-2019, 08:10 AM Thread Starter
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Any ideas? It is still going soft after a few days, with no visible signs of leakage.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #5 of 18 (permalink) Old 04-19-2019, 08:47 AM
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The master cyl could be 'bypassing'. This is the fluid bypassing the seals on the master cyl piston. Rebuild or replace is the only option for that.

Also, if you fan the brake lever to try and pump up pressure, that can pull air out of the brake fluid and release it to the system.

Hoses can also leak into the outer skin, without it dripping onto the floor. Does it lose fluid?

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
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post #6 of 18 (permalink) Old 04-28-2019, 06:08 PM Thread Starter
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No lost fluid. After sitting a day or two, it is easy to pull. Just took it for a ride. I can pull it to the handlebar without locking the front. If I rapidly pull-release-pull, it builds enough pressure to lock the front. (all my tests were done at low speeds).


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #7 of 18 (permalink) Old 04-28-2019, 06:15 PM
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When you get to being able to lock the front wheel is the lever hard, or still has some give to it?

When you get to a hard lever situation and try to pull it back does it gradually move?

'99 Vulcan "Victoria"
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post #8 of 18 (permalink) Old 04-29-2019, 07:37 AM
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How are you bleeding it? I never pump the lever. I remove the master cylinder cap, loosen the bleeder screws on the caliper (put clear plastic hoses on them to keep from making a mess) and just gravity bleed them. Pull up a chair, and holding a bottle of brake fluid, just let the calipers drip, making sure the reservoir never runs empty. After letting about a pint run through the system, make sure the reservoir is full, then get down there and tighten the bleeder screws. Make sure the master cylinder reservoir is still full. Put the cap back on. Your brakes should work fine. If they don't, then you have a mechanical problem, either in the master cylinder (probably the primary seal) or the calipers. Make sure you have no leaks anywhere. Brakes are very simple. Fluid does not compress. When you squeeze the brake lever, the primary seal on the master cylinder piston forces fluid through the line into the calipers, where it pushes the pistons out enough to apply the brakes, when you release the brake lever, it uncovers a tiny hole in the master cylinder that allows built up pressure in the system to return to the master cylinder reservoir, releasing the brakes.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
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post #9 of 18 (permalink) Old 05-03-2019, 10:52 AM Thread Starter
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Quote:
Originally Posted by VN750Rider/Jerry View Post
I remove the master cylinder cap, loosen the bleeder screws on the caliper (put clear plastic hoses on them to keep from making a mess) and just gravity bleed them. Pull up a chair, and holding a bottle of brake fluid, just let the calipers drip, making sure the reservoir never runs empty. After letting about a pint run through the system, make sure the reservoir is full, then get down there and tighten the bleeder screws. Make sure the master cylinder reservoir is still full. Put the cap back on. Your brakes should work fine.
I ran a pint through this morning. I didn't have any hoses to put on the bleeders, so I used a drip pan to collect the fluid. It made it worse! I have a coworker with a vacuum bleed system. I might give that a try.

My only hesitation to admitting a mechanical problem, is that I had no issues before flushing the brake lines. The brakes were strong. I only did it as periodic maintenance. It seems awfully coincidental that a mechanical issue would suddenly appear like that.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #10 of 18 (permalink) Old 05-03-2019, 11:02 AM
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Jason, did you get my PM?

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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