Very Soft brake petal

PilotSpike

Joined
Mar 16, 2007
Messages
8
Location
Mt. Airy, MD
Any ideas would be appreciated....

Due to the birth of a beautiful daughter and the start of a new business I had to put the ST13 in protected storage for a while, approximately 18 months. Yesterday I retrieved it, installed a new battery, check the fluids, and fired it up. Everything seems to be great with the exception of a very soft rear brake pedal. Symptoms are as follows:

Upon first use of the pedal (and only the pedal) it easily goes full travel with very little stopping power. An immediate second pump is much firmer with more brake power, and an immediate third pump the pedal is firm and functions normally. 1/4 mile down the road and this scenario repeats.

If any amount of front brake application (the lever circuit) has been made first then the brake pedal is firm and functioning on the first application as would be normally expected.

There are no ABS faults, no signs of leakage, and the fluid levels appear correct. What say you all? Is this a simple case of needing a bleed (though I don't know how air would have entered the system) or have I lost a master or secondary master cylinder somewhere?

John
 
Prob some air in the lines.. should just need to bleed them.
 
Definitely some bubbles trapped in the system, but due the longer storage period I'd rather consider a full brake fluid replacement/proper bleeding to eliminate the risk that the old fluid might have drawn humidity during the grounding time...
 
More then likely you have some moisture and air in your brake system. Bleed them properly with fresh new brake fluid and they will work like new again.
 
I hadn't had my 1300 laid-up but the rear brake started to do the exact same thing. Changed the fluids, the old fluid looked like someone with a severe bladder infection had passed it, and all is back to normal oncemore.

Change the fluid and I'm sure all will be ok.
 
PilotSpike said:
Upon first use of the pedal (and only the pedal) it easily goes full travel with very little stopping power. An immediate second pump is much firmer with more brake power, and an immediate third pump the pedal is firm and functions normally. 1/4 mile down the road and this scenario repeats.
Click to expand...
Your observations of the rear brake pedal behavior is typical of trapped air in a hydraulic brake system.

PilotSpike said:
If any amount of front brake application (the lever circuit) has been made first then the brake pedal is firm and functioning on the first application as would be normally expected.
Click to expand...
When you made this observation, was the motorcycle in motion, or was the motorcycle at rest? The difference is whether or not the SMC (Secondary Master Cylinder) had come into play. Looking at this sketch of the hydraulic brake system, it is difficult to imagine a scenario where any front brake application would affect the rear brakes unless the SMC was also in play.

If the SMC was in play (motorcycle in motion) then it's easy to imagine a scenario where the SMC applies pressure to the rear brake caliper (via the proportional valve), compressing the trapped air in the rear brake line, then when the rear brake is applied the brake pedal feels firm, just as it should. However when the rear brake is applied (without benefit of the front brake) then the trapped air will need to be compressed (pumping the brake pedal) before the brake pedal feels firm.
 
Perhaps the most important question arising from the original post is; How did so much air enter the hydraulic system in the first place?

Assuming the hydraulic system was air free before the motorcycle was put into storage, what could have occurred during the 18 months of storage that would introduce air into the system?
 
Anna'sDad said:
Assuming the hydraulic system was air free before the motorcycle was put into storage, what could have occurred during the 18 months of storage that would introduce air into the system?
Click to expand...

That's part of the mystery of ST ownership. Same thing has happened to me, along with several others on here. These bikes have to have the brake fluid flushed regularly. Or this is what happens during a long time-out. Almost seems that Mother Honda has built in a system that pumps air into the system when the brake fluid needs to be changed, forcing you to get it done. lol
 
Art Franz said:
That's part of the mystery of ST ownership. Same thing has happened to me, along with several others on here. These bikes have to have the brake fluid flushed regularly. Or this is what happens during a long time-out. Almost seems that Mother Honda has built in a system that pumps air into the system when the brake fluid needs to be changed, forcing you to get it done. lol
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Yes, it certainly seems a mystery. I find that after bleeding the brakes, it is only a matter of days before the soft pedal/lever can start to be felt again. Bleeding the brakes twice annually on a bike that rarely sits for more than a day or two seems excessive to me.
 
I flush brakes and clutch once a year using Dot 4 Synthetic. The Honda service schedule is every 12,000 miles or every two years, whichever comes first.
 
There was an earlier post where this problem persisted after a bleed, but went away after another bleed with the rear pistons pushed back in. May want to do the same.

When mine was acting same as yours, pedal would be firm with lever engaged, but would then drop if releasing the lever. Is it same with yours?








PilotSpike said:
Any ideas would be appreciated....

Due to the birth of a beautiful daughter and the start of a new business I had to put the ST13 in protected storage for a while, approximately 18 months. Yesterday I retrieved it, installed a new battery, check the fluids, and fired it up. Everything seems to be great with the exception of a very soft rear brake pedal. Symptoms are as follows:

Upon first use of the pedal (and only the pedal) it easily goes full travel with very little stopping power. An immediate second pump is much firmer with more brake power, and an immediate third pump the pedal is firm and functions normally. 1/4 mile down the road and this scenario repeats.

If any amount of front brake application (the lever circuit) has been made first then the brake pedal is firm and functioning on the first application as would be normally expected.

There are no ABS faults, no signs of leakage, and the fluid levels appear correct. What say you all? Is this a simple case of needing a bleed (though I don't know how air would have entered the system) or have I lost a master or secondary master cylinder somewhere?

John
Click to expand...
 
All of my tests were done in motion. I was thinking along the same lines as you guys with the secondary master on the front caliper taking care of compressing air before the rear pedal was applied, so good to get some validation there. Ill do a brake bleed as it is sorely overdue for it as it is. Since that seems to be the general consensus Ill take a swing at it before taking it to a stealer.

Spike
 
Anna'sDad said:
after bleeding the brakes, it is only a matter of days before the soft pedal/lever can start to be felt again.
Click to expand...

That is most certainly out of the ordinary. I'm thinking there's a weak piston seal there somewhwere, I think I'd start with the rear caliper. Here's what I think happens in your case: You park the bike with the brakes hot, and while the brake system cools, the cooling causes everything to slightly contract, causing a negative pressure in the caliper, and because one or more of the piston seals is weak (possibly from incompatible cleaning solvents/procedures), the negative pressure allows air to be 'sucked in' to where it should not be; in this case inside the brake caliper. At least that's my thought.
 
Art Franz said:
causing a negative pressure in the caliper
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Interesting idea!

The compensating port in the SMC is supposed to allow brake fluid to flow back to the caliper as the fluid cools, of course that can only occur if the SMC has not corroded to the point that the compensating port remains covered by the piston's failure to retract fully.

Yesterday I pulled the the rubber bootie off of the SMC, and lo and behold another corroded SMC. That makes it four times now that I've had to clean and rebuild the SMC.

I guess the SMC was designed more for fair weather riding then the rain, snow, slush, and salt that I habitually ride in.
 
Anna'sDad said:
The compensating port in the SMC is supposed to allow brake fluid to flow back to the caliper as the fluid cools, of course that can only occur if the SMC has not corroded to the point that the compensating port remains covered by the piston's failure to retract fully.

Yesterday I pulled the the rubber bootie off of the SMC, and lo and behold another corroded SMC. That makes it four times now that I've had to clean and rebuild the SMC.
Click to expand...

Thank you so much for getting back to me on that. That takes a bit of the mystery out of the equation! I must say I do appreciate the technology that went into designing the linked brake system and also how well it works when all's well, but, with all the troubles that have been traced back to the SMC, I sometimes wonder if we're all that much better off for it.
 
Art Franz said:
I do appreciate the technology that went into designing the linked brake system and also how well it works when all's well
Click to expand...
Oh I agree it's a clever system, even when it is not operating optimally, the motorcycle stops very well.

I think most of the problems with the SMC (certainly all of mine) can be traced back to poor weather protection. I imagine the combination of a rubber bootie instead of a proper liquid tight seal on the push-rod shaft, and the operating angle of the SMC (self filling, rather than self draining) are at the root of the problem.

If you're interested, some time ago I disassembled my SMC to document the design. Here's a link to the drawing.
 
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okmurdog said:
The Honda service schedule is every 12,000 miles or every two years, whichever comes first.
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I guess that explains why I need to bleed the brakes two or more times a year. Seems a very short interval when compared with every other vehicle I've owned, both the two wheeled, and four wheeled versions.
 
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