RC Randy Posted August 20, 2011 Share Posted August 20, 2011 Been having problem with pedal going soft on rear circuit. I have bled to Honda shop manual twice, and after about a week of daily commuting, my rear pedal slowly starts getting soft. Once it looses its hard feel at the top of the pedal, if I have first applied my front brake and then rear it is fine. But a few seconds after releasing the front lever ( such as rounding a corner) the rear pedal will sink. Don't understand this since they are separate circuits, unless my proportional valve has a problem. I ran over a quart of fluid thru it the last time, starting with a vacum bleeder, then finishing up pumping the master cylinders. Not seeing any bubbles. Not seeing any signs of leakage anywhere. Searched thru some forum posts and some have remove calipers and pushed pistons all the way out and started from there? Anyone have any ideas? And no, I dont want to de-link. And it is my 99XX with 65K. Quote Link to comment Share on other sites More sharing options...
Zero Knievel Posted August 21, 2011 Share Posted August 21, 2011 I wish I had a brilliant answer for you. From when I replaced my lines on the XX a couple years back, I noticed the left-front caliper has a piston that actuates part of the rear brake. When the front brakes grab, the inclination to turn forward with the wheel compresses the piston and sends pressure to the rear. Ergo, when you release the front brakes, you are LOOSING the pressure this line is applying to the rear and the pedal is the only actuator sending pressure to the rear brake. To get the same effect, you must press harder. IIRC, the rear pedal likewise sends some pressure to one of the pistons on one of the front calipers, but forget if I discovered that the only link was this one piston on one caliper powered by that contraption on the front brake caliper. I do know that I did a lot of sucking and pumping (no dirty jokes please) to be sure all the air was out of the system. It took a lot of pumps to pressurize the system. I've not noticed any "softness" but here is my guess.... Brake systems tend to equalize pressure. Pump your front or rear brake several times and the brakes will get real firm. Let them sit a while and the pressure eventually equalizes back into the reservoir. Then, if you try to use the brakes, they'll feel noticeably soft compared to before. The brakes still have power, but instead of hard and firm at the moment you apply them, you are putting a decent grip on the lever to get maximum force. If it's not supposed to be that way, perhaps I have an undiagnosed problem as well. My XX stops when I want it to, but I too have noticed that it feels softer AFTER you get it all sorted out, but this is why I suppose it happens. I have the same issue in my truck, and I know there's no air in the system. Feels great when you first flush the fluid with new stuff. In a few weeks, it feels like it always did. Quote Link to comment Share on other sites More sharing options...
ironmike Posted August 21, 2011 Share Posted August 21, 2011 Had the same issue with my XX and my GL1800 (same linked brake system). It can be enormously frustrating, but the actual physics are pretty basic. If the pedal is soft, the problem is, as you suspect, a trace of air in the system. Liquid cannot compress, but air can and will. The most likely locations are in the vicinity of the left front caliper auxiliary master cylinder - the highest point of the rear system. The only fix that I've found that works is to repeatedly bleed the system following the shop manual instructions religiously (the proper sequence is critical). In my experience, three to five attempts usually usually get's 'er done. Patience is the key. Sometimes waiting a day between bleeding sessions helps, but that may be more psychologically soothing than mechanically effective. Quote Link to comment Share on other sites More sharing options...
blackhawkxx Posted August 21, 2011 Share Posted August 21, 2011 Sometimes waiting a day between bleeding sessions helps, but that may be more psychologically soothing than mechanically effective. Or riding it between sessions could help move the air so it can be bled. Quote Link to comment Share on other sites More sharing options...
Banshee Posted August 22, 2011 Share Posted August 22, 2011 The pedal operates the center pistons on the calipers. It operates the rear and the front right directly. The front left is operated via a 'delay' valve. As the front left comes on, it applies pressure to the rear outer pistons via the secondary master on the fork leg. All the outer pistons are supplied by the front reservoir, the rear only supplies the center pistons. If you were pressing hard enough on all the brakes to compress the pad material, when you release the front brakes, the pads would push back & the pedal would get harder or push up, not get softer. Other than air, I have no idea what might cause what you describe. Has a pad worn unevenly due to a stuck piston on a calliper, perhaps? Or a sticky floating pin, maybe? Quote Link to comment Share on other sites More sharing options...
Zero Knievel Posted August 22, 2011 Share Posted August 22, 2011 The pedal operates the center pistons on the calipers. It operates the rear and the front right directly. The front left is operated via a 'delay' valve. As the front left comes on, it applies pressure to the rear outer pistons via the secondary master on the fork leg. All the outer pistons are supplied by the front reservoir, the rear only supplies the center pistons. If you were pressing hard enough on all the brakes to compress the pad material, when you release the front brakes, the pads would push back & the pedal would get harder or push up, not get softer. Other than air, I have no idea what might cause what you describe. Has a pad worn unevenly due to a stuck piston on a calliper, perhaps? Or a sticky floating pin, maybe? Life would be so much easier if they'd perfect transparent brake lines. Quote Link to comment Share on other sites More sharing options...
jon haney Posted August 22, 2011 Share Posted August 22, 2011 I'm going to say it anyway. If you want a firm rear pedal, DE-LINK. It won't take much longer to do than bleeding the stock system and you only have to buy a two line hose kit for the front. You'll also lose 2-3 lbs. of metal in the process. Quote Link to comment Share on other sites More sharing options...
Para045 Posted September 19, 2011 Share Posted September 19, 2011 The pedal operates the centre pistons on the calipers. It operates the rear and the front right directly. The front left is operated via a 'delay' valve. As the front left comes on, it applies pressure to the rear outer pistons via the secondary master on the fork leg. All the outer pistons are supplied by the front reservoir, the rear only supplies the center pistons. If you were pressing hard enough on all the brakes to compress the pad material, when you release the front brakes, the pads would push back & the pedal would get harder or push up, not get softer. Other than air, I have no idea what might cause what you describe. Has a pad worn unevenly due to a stuck piston on a calliper, perhaps? Or a sticky floating pin, maybe? +1 I reckon there is still air in the system somewhere, most likely as IM said the aux MC :icon_think: I had problems years ago when I had an old 78 F100 4WD, I got the rear converted to disc brakes which worked brilliantly until I replaced the lines with braided lines and then I seemed to have no rear brakes :icon_wall: I tried everything I could, bled it a dozen times, removed and checked the secondary MC/balancer for the front/rear and even took it to a specialised brake shop who pressure bled it and still no joy Even tried putting a brake bias adjuster in-line from the front thinking it might help but no :icon_doh: I then removed that and refitted the copper line from front to back and the brakes came good The only thing I could figure is that somewhere along the line there was an air bubble trapped in the SMC Quote Link to comment Share on other sites More sharing options...
cbrxxquad Posted September 19, 2011 Share Posted September 19, 2011 A possible solution, is to remove the calipers and push the pistons back in as far as they will go, pushing all the fluid back in to the res. Hang back on the disk and pump up again till the pads contact the disk. repeat bleeding procedure, and pushing the pads back in again till all the air is removed. I find that in difficult jobs the moving the calipers so the bleeder is down when I push the pads in will move the trapped air back up the line to the res and out of the caliper. Helps to move a larger volume of fluid back and forth to displace the trapped air. Quote Link to comment Share on other sites More sharing options...
blackhawkxx Posted September 19, 2011 Share Posted September 19, 2011 A possible solution, is to remove the calipers and push the pistons back in as far as they will go, pushing all the fluid back in to the res. Hang back on the disk and pump up again till the pads contact the disk. repeat bleeding procedure, and pushing the pads back in again till all the air is removed. Great idea. Quote Link to comment Share on other sites More sharing options...
RC Randy Posted October 9, 2011 Author Share Posted October 9, 2011 Well after almost running the world out of DOT 4 brake fluid I finally have a good hard rear pedal circuit. Most of the problem stemmed from not putting enough pressure on the rear pedal, compressing the pistons and pads against the rotors. I of course was using my hand, and putting pretty firm pressure on the pedal, but once I started bearing down on it with a whole lot of force, I got some bubbles out, and it firmed up. I also quit using the vacuum bleeder, and just used the pedal. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.