Bizarre Soft Brake Pedal

So i was changing the front pads on my civic and when i was finished, suddenly the brake pedal was a lot softer than it used to be and i could now push it to the floor with moderate pressure. when i was pushing the caliper cylinder back i thought i saw a bubble or two and a tiny bit of brake fluid leak out so i bled the system three times (in one case, i totally emptied all the fluid and refilled it with fresh DOT4), but it hasnt stiffened up. Its driveable... if i slam on the brakes it will barely lock the front wheels before the pedal hits the floor.

I have a few theories and im wondering what you guys think.

1) Bad Caliper Cylinder - Could that little bit of fluid leak i saw be enough to cause a very soft pedal without any noticible Brake Fluid loss? ive checked for wetness while parked with the pedal pushed down and found nothing...my gut tells me that was a one time dribble from pushing the cylinder back.

2) Bad Master Cylinder - If I push the pedal to the floor for a long time, it still retains whatever pressure it has there. pumping seems to have little effect. so im thinking that might not be it.

3) Bad Brake Booster - I do see the engine rpm's rise and fall while applying and releasing the brakes, which as ive been reading here, is evidence of a straining or leaking Booster. But that would only make my pedal feel harder right? Perhaps that rpm fluctation is normal.

4) Air is somehow still in the lines / getting in the lines. I dont know how that would be possible, but as far as i can tell, all the symptoms fit air in the lines. it maintains pressure.... just not enough and with a very squishy feel.

5) Bad Pads - It all started when i installed the new pads... however they are Raybestos Quiet Stop Ceramics, just like the last two sets. Exactly the same pads... Can a brake pad be warped, bent, or otherwise somehow flexible causing that lack of firmness?

sorry for writing a book here... im just stumped.

sounds like air to me. how did you bleed them the first time? maybe your friend that was pushing your pedal down didnt hold it long enough and let air back into the system?

Try bench bleeding the master cylinder first, before bleeding at each wheel.

hmm... well after another bleeding i feel like theres no way there can be air int there. if i stomp the brake pedal, i can lock the wheels... gradual application results in a sinking pedal after a second or so until it evens out. perhaps the master cylinder seals are bad and can only hold so much pressure? so i'll try replacing the master cylinder tomorrow. they're relatively cheap.

Yeah, sounds like the master cylinder is bad.

well i replaced the master cylinder and re-bled the brake system twice more - once with a vacuum bleeder and then again with my dad at the brake pedal. absolutely no air was coming out and i put a solid quart of brake fluid through the system.

im absolutely astounded that i can have a seemingly perfect hydrolic system and still have a soft pedal and not enough braking force to lock wheels. The rear brakes are only a few thousand miles old and properly adjusted. The booster seems plenty strong if im able to floor the pedal... theres no fluid loss from the reseivour over time...

this started when i changed my brake pads, but pads are pads right? they were identical to the previous set (Raybestos Quiet Stop). im absolutely stumped.

As long as you didn't open up any lines you should be ok.
After changing the pads you should give the pedal a few short taps about halfway till you get a stiff pedal.
If you accidentally went beyond 3/4 this is where you could of introduced air in the system.

Bleed the master again following the directions that came with it, then start bleeding each wheel starting with the wheel furthest away from the master cylinder RR/LR/RF/LF.

Good luck!

im almost 100% certain at this point that its not a hydraulic issue. The new master cylinder is in (and i had no trouble with the master cylinder installation and brake bleed i did on my dad's honda accord). I've since done 5 bleeds and no air bubbles come from the lines. i can tap the brake a short distance all i want, but it doesnt stiffen up.

My current theory is this:
1) You're probably right...I pressed the pedal too far after installing the pads, letting air in. - and thats a separate issue. once that air was bled out, that part of the problem was solved.
2) I installed defective pads or my new pads were unable to contour to the old rotors. The brake pedal felt fine when it was parked before the brake pads were replaced.... and thats the only change i made.

so i went ahead and purchased new pads (and rotors since my old ones were thoroughly damaged anyhow... i can feel grooves and unevenness in the surface around the rotor with my finger - they're impressive). heck... maybe thats it. the new pads cant make full contact due to enormous surface variations in the rotor and then bend and compress under the strain of being crushed into an uneven surface, sinking the pedal and reducing contact area/braking power. The old pads were worn to match those circular grooves.

Does that sound like a possible explaination? (i suppose we'll find out if it works when i install them tomorrow too)


hahaha ps i also see hairline cracks radiating from the center to the edge in the rotor surface. These are utterly finished. im keeping them as souvenirs.

I've installed pads on cars with rotors with slight grooves before and have never had a soft pedal afterward. I highly doubt that the pads and rotors will be the issue.
The front calipers will always have the pads just slightly off the caliper after releasing the brake pedal. So once the pedal is pressed the pad comes directly into contact with the rotors.
Usually the rear drum brakes after time will cause a softer pedal if the self adjusters are not working properly. A soft pedal as you describe is definatley the sign of air in the system.

1. Are you getting fluid out of the the bleeder screws?

2. Did you bench bleed the master cylinder off the car?

3. Are you bleeding the brakes with a tube attached to the bleeder screw and the end of the tube submerged in a bottle of brake fluid. (I use a small cooking oil bottle with a wire that I can hang near the wheel for this.)

4. While bleeding once again short strokes about half way till you stop seeing bubbles in the clear bottle. Once they stop press the pedal one last time and hold the pedal there then close the bleeder screw.

5. Keep an eye on the master cylinder and never let it become empty or you will have to bench bleed it once again.

Once again good luck to ya.

wow you guys didnt think any thing about brake line expansion. even lines a few years old can jsut expand and cause well basicly no directed pressure. this is why most tuners run stainless lines. maybe this might help you out.

Yes dangstangs, i had clean, bubble-free fluid coming out every time it was bled. the master cylinder was bench bled, the brakes with a tube in fluid. ive successfully bled several times before, which is why i was confused as to what could have gone wrong this time... or the last 5 times for that matter.

Well believe it or not... its now working.
I finished installing the new pads and rotors. I pretty much didnt touch the hydraulic system today except that i pushed the caliper pistons back to install the new pads and had to remove some fluid from the reservoir to compensate.

The front edges of my pads were literally crumbling off and there were several very deep scores around the old rotors.

after that i just gave a few short half-pumps of the pedal and rolled it out of the garage. and what do you know... the pedal is a lot stiffer and the car stops faster with the same braking pressure. I took it out for about 90 miles of easy country driving with a few light-moderate speed stops and it feels very much smoother and firmer both while braking and in normal driving (i think those rotors were rubbing the pads at speed too). anyhow... its fixed now!

Glad to hear that you got it fixed. I still would have never guessed rotors or pads, but hey I'll just add it to the book of common problem weird solution.

yeah, cheapo pads create a lot of gas when used which can lead to brake fade

I knew it. Always replace or resurface rotors when replacing pads. :thumbsup:

haha yeah lesson learned... unless the rotors look really perfect, they're outta there! The new ones, made by Raybestos, were 18.99 each i think... Shucks wanted $15 a piece to resurface my old ones (which im sure were beyond the point of repair). in any case, it seems hardly worth resurfacing regular rotors. The car hasnt driven this smoothly in many thousands of miles.

haha yeah lesson learned... unless the rotors look really perfect, they're outta there! The new ones, made by Raybestos, were 18.99 each i think... Shucks wanted $15 a piece to resurface my old ones (which im sure were beyond the point of repair). in any case, it seems hardly worth resurfacing regular rotors. The car hasnt driven this smoothly in many thousands of miles.

Rotors are basically throwaways these days. Theres not much meat to them anymore your lucky to get one cut out of them let alone two and with some new rotor prices new is the way to go.