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Rear brake woes... air in the system AGAIN?!


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I rebuilt all the calipers, flushed/bled the system once. It felt okay, but got really spongy over a few weeks... full pedal travel and nothing happens spongy. Figured I didn't bleed the system properly.

 

So, I bought speed bleeders and thoroughly re-bled the system as per the service manual... making sure I followed the exact sequence including removing the front left caliper and orienting the slave master cylinder at the right angle. A ton of air came out of the PCV. Pedal felt fantastic immediately afterwards, braking performance was improved, and I was happy as a clam.

 

That was about a month ago. Spent the last couple weeks replacing all the coolant lines and got her back together for a test ride.

 

Rear pedal is spongy again!!! What gives?

 

I'm suspecting a leak someplace where air is getting into the system, but there is no loss of fluid at the rear reservoir nor any wet spots.

 

Any suggestions?

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  • Member Contributer

Sounds very similar to an issue I had with a car of mine. I ended up replacing the master cylinder as it had failed.

If fluid is bypassing the internal O-rings in the master it will give that feeling of sponginess. 

 

I have also seen cases in the car world where the flex lines are ballooning out

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  • 1 month later...
  • Member Contributer

I rebuilt the brakes on my ‘98 last year. The rear brake had started to “self-apply”! I found some water in the middle piston area of the rear caliper which is activated by the pivoting left front slave cylinder when the front brakes are applied. As for bleeding- after refilling the system (I have every type of bleeding system- mityvac, ebc air powered,  big syringe for reverse bleeding) and still having a soft pedal, I started forcing the rear caliper pistons back into the caliber body and then manually exercising the left front slave cylinder (the one on the pivoting mount) while bleeding the rear (remember to unbolt it and hold it upside down so the bleeder nipples are pointing up. 

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Water... I hope that was before fresh fluid, but shouldn't be now.... I give a little flush and bleed every year, keeps things optimal.  When I got my Gen5 a few months ago, it needed flush and bleed because the fluid was dark and ugly, clutch too.  I did not pump the SMC, but held it up while bleeding the PCV, then the rear, just followed the manual.  Could not get teeny bubbles out of the rear no matter what, so hung the rear caliper up high (above PCV height), put a bungie on the brake pedal to hold it down overnight.  This forces air to the highest points, can do same with clutch and front brakes (zip tie to handlebar) after a bleed, any air should travel up overnight into the reservoir.

Next morning, bled the PCV, no air, bled rear, a small bubble, then no air.  Now that it's bled, will go back to annual flush/bleed, will take very little.

I should mention I replaced all the bleeders with SpeedBleeders, which have a thread sealant to prevent air going in the threads.  PCV has a unique bleeder screw with an o-ring.. no SpeedBleeder there.  MightyVac and syringe OK to get things primed, but after that I prefer the old fashioned way.  Hope something similar works for you.

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After 20 years, I wasn’t surprised about what I found in the rear caliper. You try an maintain your bikes as well as possible but time passes by and a couple of years have gone by since you replaced fluids. 
The braking system is a bit complicated and I think you have to think outside the box when you get into the bleeding of it. I replaced all the seals in the rear caliper- unfortunately there are some components which are no longer available and I was hoping they were not the problem. There was one component, perhaps the PCV, that someone spoke of having a screen which was clogged with debris. Did not get into that. But after a couple attempts of “reverse bleeding” it seemed ok.

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Before I purchased my current 5th gen I had test ridden 2 others and all of them had spongy Rear brakes, as does my current 5th gen.

I always figured it was a 5th gen thing, 

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I was expecting some sponge based on what I was reading, but I haven't got a problem that I know of.  Back brakes by themselves could maybe be better, but in reality, don't want them too strong either as I have no ABS on this bike.  I upgraded the fork springs, so leff front end dive, and I think that helps change the feel a bit, it would take some significant dive now to move the SMC and I think it moves less now.

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