Civic Brake Prob

Over the past few weeks the brake pedal on my 95 Civic Ex (195,000 miles)with ABS would sometimes sink a little at stoplights and statrted feeling a little funny the first time I applied them lightly in the
morning. My first thought was to bleed the brakes. And, while bleeding the brake pedal yesterday, I lost all pressure and the brake pedal consistently went to the floor. I had placed a piece of wood under the pedal so the brake stroke would be the same. The search feature of this wonderful forum led me to purchasing a master cylinder. The problem is now fixed. What I don't understand is how a master cylinder could fail so completely because you have two seperate hydraulic brake systems? When my pedal went to the foor, it meant I had lost both of these brake systems. There were no external leaks, and I never had to add brake fluid. Had one of my brake systems failed a while back and I was not aware of it? Thanks in advace of any thoughts on the issue.
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The piece of wood won't prevent a person from pushing so much brake fluid out of the system that air is introduced via the master cylinder reservoir becoming too low.
ISTM that's what happened, and the air entering via the reservoir enters both "sides" of the brake system. I messed up a couple of years ago during a routine brake system drain/fill/bleed and had the same thing happen (pedal went to the floor while bleeding brakes; but I /know/ I let the reservoir get too low and introduced air). I re-did the bleed, and all was well. (There were and are no problems with my master cylinder.)

I think you fixed two problems: An internal leak in the master cylinder (fixed by replacing it completely) and air introduced to the system (fixed by doing a thorough bleed after replacing the old master cylinder).

I think it meant you introduced air into both systems, in this instance.

You did add brake fluid at some point during all this, didn't you?

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Elle, what you said makes perfect sense, except I never let that big master cylinder get below half full. When bleeding the brakes with a short stroke, because of the wood, I was especially careful not to introduce air. At that point, I only thought I had air in my system. I thought I may have introduced air months ago when I used a one person brake bleed setup. Never got any air during the bleed. This time there were two of us doing the bleed. When It went to floor, I thought this could not be possible. Fluid level was fine and by continuing to pump the brakes, pedal came up and then we lost it again. Yes, I have also introduced air into a brake system before on a different car by letting reservior get too low. Did not want to repeat that mistake when I was trying to get air out of the brake lines. Thanks for your input.
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If the master cylinder piston seals fail,fluid seeps around the seal instead of being pushed into the brake lines,thus the pedal moves,but no braking;bacause no pressure is applied to the hydraulic fluid. I've read that moisture can cause corrosion in the aluminum MC bore and that erodes the seals(neoprene?),affecting both sides of the system.
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Jim Yanik
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Jim Yanik wrote:

the best explanation of the system is here:
http://www.tegger.com/hondafaq/mastercylinderreplace/howworks.html
[the rod inside the item labeled "second circuit return spring" is not a fixed rod so only hydraulic pressure moved the fist circuit piston, not mechanical.]
there's not really a "both sides" - they both draw fluid from a common reservoir. the rubber's not affected by water. water contaminated fluid however does set up differential corrosion between the steep pistons and aluminum bore which in turn can abrade the rubber, and contaminated fluid from oil vapor, etc can affect the rubber... but it's all academic. keep the system regularly flushed, and replace/re-seal every 10 years and you'll be fine.
[i have 17 years on my 89, and i think it's only now thinking of flaking out.]
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duckbill wrote:

the seals wear and leak. fluid passes past the seals - just like a shock absorber. it's real simple. the floating secondary piston doesn't make any difference to this as all seals usually wear at the same rate. the only thing to worry about is replacing and bleeding, which you have done, and taking the care to flush the system with fresh brake fluid every year or two, depending on climate.
in the future, if you're bleeding a new master cylinder, don't dick about with the piece of wood. the pistons are designed for full travel, and failure to exercise that on some vehicles means you'll never succeed in getting all the air out. the wood theory comes from old wives tales about "i bled the system and the cylinder failed a couple of weeks later". reality is, the seal was shot but swollen due to contaminated fluid, therefore it kinda-sorta held in there. clean fluid re-shrank the seal, which would have happened regardless of bleeding method, and it started to leak. not changing the fluid may have allowed it to survive a little longer, but it was on its way out anyway. moral of the story: change the seals/cylinder and bleed properly. if you expose additional problems, it's a good thing - this is a critical safety system after all...
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Some good points Jim, thanks; now if I could just figue out how to change the fluid in the ABS accumulator without the required Honda special tool. I have replaced the fluid in the ABS resovior. Again, thanks in advance!
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duckbill wrote:

you pretty much have it covered without special tools. i guess if you wanted to be really anal, you could take the car out onto some gravel and exercise the abs for a bit, then change the fluids again. brake fluid is cheap.
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What do dealers typically charge for brake system flushing/fluid replacement?
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Probably off course on this one, but wanted to make sure... you have been using Honda brake fluid? Other brands may not be compatible, in which case you can get that happening. It happened to me when I had a Lotus with the old Girling brakes and let the parts guy at Western Auto convince me all brake fluids were the same. The fluid attacked the seals starting with the secondary seal between the two halves of the cylinder... it had fluid on both sides. When the primary seal failed the pedal went right to the floor.
Otherwise, I agree replacing the master cylinder was the cure one way or the other.
Mike
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Thanks Elle, Jim Y, Jim B, Michael, and Edokamoto. And a special thanks for Tegger for the awsome master cylinder information. I did take it out on the gravel and exercise the abs. I changed the brake fluid in the reservoir tank again because it was now looking very (amber) used Bleeding all the brakes again will have to wait. Happy 4th to all.
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When I work at the shop where I work part time we have had a number of master cylinders or slave cylinders fail all of a sudden while bleeding or changing brake parts and we (me and the boss) think it is because when you bleed a brake since there is no fluid to stop piston travel the piston goes further down the bore than it usually does and dirt and corrosion might have accumulated there to rip up the seals. Like 3 months back I changed an old toyota camry rear brakes and we pressed on brake pedal and started to adjust it. Then we noticed it leaked a huge leak from cylinder, had to change cylinder, it was not leaking at all before. Only difference was brake unadjusted so pistons in brake cylinder traveled further in bore to dirty rusty surface that ripped up seals. Also I just bled my clutch slave cylinder, was alone so I had to use clear tube in glass bottle technique, and I filled clutch master about 7 times and pumped out fluid. Boy you should have seen all the black stuff in the bottle. Also if you understand how a brake master cylinder works you will see how if one circuit fails the pedal will go further to floor. In a typical master you have 2 pistons one for each circuit. In normal operation when you press on pedal it pushes rear piston forward, The rear seal will push fluid lightly and push front piston forward, then front piston and rear piston will start to apply brakes. If front piston circuit loses pressure then front piston will go into bore till the metal part of piston hits end of bore and rear piston will also travel further (remember there is a rear facing seal on front piston. ) Now if rear piston circuit loses pressure then the rear piston will go forward till the front part of it hits the front piston and pushes it forward where good forward piston circuit pressure will stop the front piston and rear piston. If both circuits fail the when you push the pedal then the rear piston will run into the front piston and the front piston will run into the end of the bore( in this case the pedal will travel the furthest toward the floor.
duckbill wrote:

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snipped-for-privacy@netzero.net wrote:

potentially true, but two things:
1. if you flush the fluid regularly, like it says in the book, this doesn't happen. tegger's obsessed by this stuff more than any normal person, and his cylinder was absolutely /pristine/. but his seals still wore out.
2. the system /has/ to be able to function in an emergency stop without seal failure. if using the full bore of the cylinder unearths such an issue, it's a /good/ thing, not bad.

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