Soft brake pedal :(

I took my E46 M3 to Watkins Glen last week for a 2 day DE. After the first session the brake pedal got somewhat soft from what was reasonably solid pre-track, and after the second session it was very soft. I had the DSC turned on for those two sessions and didn't run that for the remainder of the 2 days - it was triggering almost constantly for those two sessions though and because it doesn't like any hint of yaw at all. Unfortunately no amount of bleeding of flushing has gotten the pedal back. Basic two-man bleeding, pressure flushing, more two-man bleeding, nothing helped. The front pads (KFP golds) were very very worn by the end of the event but not to the point where the pad sensor triggered, and both front and rear pads look cooked. Tonight I re-bled with a combination of a vacuum bleeder and two-man bleeding, and I also used INPA to run the ABS pump while bleeding each wheel. Still soft I intend to get some new pads in hopefully this weekend (PFC06 or DTC60) and run a couple quarts of fluid through the system with the old fashioned two-man system. Any other ideas to get a hard pedal back? I need a good pedal so I can practice my heel-toe before my next DE in late Sept.

MOTIVE BLEEDER

Get yourself a Motive Power Bleeder and a couple liters of ATE 200 or Super Blue fluid. Performance Friction 06 pads are very good but may be a bit much for street tires. I use 'em w/ Toyo RA1s. Hawk HT10s are pretty good as well.

The Motive bleeder is awesome. I can swap in track pads and wheels and bleed all 4 corners in about 45 minutes total, maybe less.

The soft pedal was from when the fluid boiled in the calipers. It shouldn't be hard to bleed out 'cause it's out at the corners.

A bled a freshly built motorcycle brake system last week - all rebuilt M/Cs and calipers, new virgin lines. I used a little vacuum pump (a tiny Gast airbrush compressor that's also vacuum on the non-blow side) on the bleeder nipple to pull the fluid through, then pumped and bled the conventional way. Yesterday, with my ace brake bleeding assistant, my wife, I bled the system on the Radical on which I just installed 2 new Wilwood .75" M/Cs - It took maybe 40 or so stroke, hold, release and wait for bubble in the reservoir, repeat cycles before it was really pushing fluid, then it bled easily. Still, I went around the car twice. The pedal is like granite now.

So get a Motive and it'll be done in minutes. Seriously.

I was running ATE Typ 200 when I boiled the fluid with the stupid DSC. After the pedal went soft at the track I did a quick go-around that didn't dislodge any major bubbles, then I had them flushed with a full quart with a pressure bleeder like the Motive (but using air pressure from the front tire - trick!), then another go-around at all 4 corners with the two man method, no bubbles found, but still a soft pedal. I may get the Motive yet but for now I'm going to just flush the bejesus out of them the old fashioned way.

On pad selection, my instructor recommended DTC70s even with my street tires (RE11s) - I intend to go with slicks next spring. I figured the PFC06/DTC60 would be a little better choice, both from a pad longevity and slightly less torque standpoint.

Following up, I put a liter of Super Blue through the system and no change. I finally got around to swapping in the new PFC06 pads today and found what's likely 90% of the problem - the passenger front pads were tapered quite a bit giving the long pedal. It's like the caliper was not mounted on the same plane as the rotor. Anyway, once a bit of heat is into the 06s the pedal is nice and firm.

All is not perfect however - it will still slowly drop if I push and hold while sitting still and it makes a squishy noise when the pedal is pressed. The squish noise sounds like it's coming from the pedal/firewal area. I'm wondering if I have bad seals in the master or an issue with the brake booster - any ideas?

This is probably super obvious but, you aren't losing any fluid are you?

ShadowSix wrote: This is probably super obvious but, you aren't losing any fluid are you?

No fluid loss or leaks anywhere.

If it sinks, that means fluid's going around a piston. If it were going around a caliper piston, it'd wind up on the ground. But in a master cylinder, fluid getting around a piston just winds up in the reservoir.

I'd guess you have a bad master cylinder.

The only thing that makes me think booster and not master is that if I stand on the pedal with the car not running it's solid and doesn't drop. Booster vacuum check valve perhaps?

I have it in my head that the only reason the pedal can drop with continued pressure is that fluid is leaving the captive space between the m/c pistons and caliper pistons.

I could be wrong, but I don't see how vacuum's going to have any effect on the pedal's ability to stand firm. I tend to think the reason the pedal's firm when not running is that you can't put enough force on the M/C to get it to leak or leak fast enough to see the drop.

your problem sounds very much like what I've been going through for the past couple of yrs... the itty-bitty rotors ( in my case, maybe not yours) and the tiny single piston calipers of the CRX have 2 problems .... one they're so small they get heat soaked long before the fluid has a chance to "boil"... which leads to a soft pedal .... cooling and flushing the air bubbles helps...

the other problem which sounds more like what you're going through is the floating caliper doesn't let the pads wear evenly ... when new I have great brakes... as they wear I end up using my left foot to "pump" ( not so much air) the pads back into "square" contact with the rotor .... I start pumping about the 600 mark going into a corner .. as soon as I feel the pedal get firm I'm ready for the corner.... and again I have great brakes.... this is pretty much the procedure after the first day with a set of pads... this seems to work all the way to the rivets ..... might be a stop gap solution for you ...

my solution is going to larger rotors and fixed 2 piston calipers ( Wilwood )

I had a similar problem with the 924s and it was the floating calipers hanging up. I would get about half pad ware with normal brake's and then things got really spongy.

Don't rule out the master. The sinking pedal with out fluid loss is the key here.

The simple answer is don't use as much brake.... . No really. I am not trying to be a #$%@#. I bet you will be faster. It will make for what feels like a slower boring lap but I bet you will be faster. I clicked off my fastest laps at NHIS while just relaxing and not really trying. The old go slow to be fast thing.

Way back when I would use a set of pads in a single day (4-5 20-30 min sessions) Now I can do 2-3 2 day events before I need pads. I am allot faster now as well.

I'm leaning towards an internally bypassing master but at $300+ it's not something I want to just throw at the car without more confidence in my diagnosis. I just don't understand why the pedal is rock hard with the car off and the vacuum used up out of the booster but slowly sinks if I stand on it while the car is running.

In reply to pigeon:

I suspect this: The master seals aren't nonexistent, they're just poor. So, it follows that they'll hold under X psi, but perhaps not X + Y psi.

You've probably noticed that even when the braking system is in perfect order, a pedal that feels like granite with no vacuum feels a good deal softer when the engine's running. That's because the force the booster is adding is substantial, making Y a significant value. (I just googled for an actual value in terms force but couldn't find anything quickly)

In any case, I think it's a good call waiting for confirmation before chucking three bills at it. I just think the symptoms you're seeing with and without vacuum are quite sane given the above.

ransom wrote: In reply to pigeon: I suspect this: The master seals aren't nonexistent, they're just poor. So, it follows that they'll hold under X psi, but perhaps not X + Y psi. You've probably noticed that even when the braking system is in perfect order, a pedal that feels like granite with no vacuum feels a good deal softer when the engine's running. That's because the force the booster is adding is substantial, making Y a significant value. (I just googled for an actual value in terms force but couldn't find anything quickly) In any case, I think it's a good call waiting for confirmation before chucking three bills at it. I just think the symptoms you're seeing with and without vacuum are quite sane given the above.

i'm going to go with ransom on this. typical range of boost ratios on passenger cars is 3.5:1 up to 7:1, so yeah, there's going to be a lot more force going into the MC when the booster is charged.

This is a typical problem on track driven M3's. The E36's have it worse. Many people have spent lots of time and money chasing it, but never solve the issue.

In your case, I'd make sure you're cycling the ABS while you're bleeding. You may need to take the car to a shop who has the proper computer to do that.

Then I'd check your wheel bearings...