Front Brake Spongy - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 5 (permalink) Old 04-06-2019, 01:18 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 Vulcan 750 Purchased 4/12/18 @ 17805.9 miles $800
Purchased stock except for K&N Filters. Wrecked once
20969.2 miles & counting

Completed modifications:
Removed mufflers, running straight out of goats belly
Pick-up coil re-gapped to .020"
Compufire 55402 r/r series regulator
USB Outlet + Voltmeter
Repainted Metallic Blue w/ new tank bages

Planned modifications:
Ear Shave (without coasters)
Custom LED turn signals
Windshield
Highway Pegs
(Possible) ignition advance
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post #2 of 5 (permalink) Old 04-06-2019, 09:01 PM
GDI
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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 5 (permalink) Old 04-14-2019, 06:37 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 Vulcan 750 Purchased 4/12/18 @ 17805.9 miles $800
Purchased stock except for K&N Filters. Wrecked once
20969.2 miles & counting

Completed modifications:
Removed mufflers, running straight out of goats belly
Pick-up coil re-gapped to .020"
Compufire 55402 r/r series regulator
USB Outlet + Voltmeter
Repainted Metallic Blue w/ new tank bages

Planned modifications:
Ear Shave (without coasters)
Custom LED turn signals
Windshield
Highway Pegs
(Possible) ignition advance
Jason Pittenger is online now  
 
post #4 of 5 (permalink) Old Today, 07:10 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 Vulcan 750 Purchased 4/12/18 @ 17805.9 miles $800
Purchased stock except for K&N Filters. Wrecked once
20969.2 miles & counting

Completed modifications:
Removed mufflers, running straight out of goats belly
Pick-up coil re-gapped to .020"
Compufire 55402 r/r series regulator
USB Outlet + Voltmeter
Repainted Metallic Blue w/ new tank bages

Planned modifications:
Ear Shave (without coasters)
Custom LED turn signals
Windshield
Highway Pegs
(Possible) ignition advance
Jason Pittenger is online now  
post #5 of 5 (permalink) Old Today, 07:47 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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