Soggy Brake Pedal
Discussion
Since receiving my car last week, I've felt a little uneasy about a soggy brake pedal. Especially the first time activated and even after not braking for awhile after driving the brake pedal travels quite a bit (2-3 inches?) before biting. Pumping the pedal gets it to firm up such that it bites after perhaps 3/4" of pedal travel. Still not like a Ferrari, but at least confidence instilling. Depressing the brake pedal a couple of inches before any noticeable braking action is a bit unnerving and I can't believe that this is within design specs.
The obvious diagnosis is air in the brake lines. The car is 8 or 9 years old and I don't know how long the fluid has been in there so I did a complete flush with ATE SuperBlue and a Motiv PowerBleeder. No change. I flushed the lines again and verified blue fluid and no bubbles at all 4 corners (working from farthest from reservoir to closest to reservoir). Still no change. Perhaps even worse?
I put a little more fluid in the reservoir to the spec that is printed on the reservoir -- to base of neck. Putting the cap back on, I noticed that the little black "AP" coverlet on the cap had come off. The cap, with its two wires and a float bob integrated within apparently has a low-fluid sensor. Perhaps this is the cause of my soggy pedal? Is the little rubber caplet supposed to be attached with any sort of adhesive?
The obvious diagnosis is air in the brake lines. The car is 8 or 9 years old and I don't know how long the fluid has been in there so I did a complete flush with ATE SuperBlue and a Motiv PowerBleeder. No change. I flushed the lines again and verified blue fluid and no bubbles at all 4 corners (working from farthest from reservoir to closest to reservoir). Still no change. Perhaps even worse?
I put a little more fluid in the reservoir to the spec that is printed on the reservoir -- to base of neck. Putting the cap back on, I noticed that the little black "AP" coverlet on the cap had come off. The cap, with its two wires and a float bob integrated within apparently has a low-fluid sensor. Perhaps this is the cause of my soggy pedal? Is the little rubber caplet supposed to be attached with any sort of adhesive?
Well, at this point in time I have flushed the brake system 3 times (both inside and outside calipers, in that order) and used two different DOT 4 fluids. No real change. After driving it for a couple of hundred miles now, I can characterize the issue as a brake pedal that is alternatively soggy (with an inch or two of travel before engagment) or firm the first time the brake pedal is depressed. Whether I get a firm or soggy pedal seems to be somewhat related to how long it has been since the prior use of the brakes. The longer it has been since the last braking activity, the more likely I'll have a soggy pedal. Uniformly, the next pedal press (soon after the first) is firm. With a firm pedal, continuous pressure on the brake pedal does not result in any change -- i.e. does not go to the floor over time.
Although this still sounds like air in the system, I'd be astounded if 3 flushes with 2 different fluids is not sufficient to adequately bleed the system (with a Motiv PowerBleeder). So now I'm considering other possibilities.
Sounds like I need to find the M10 x 1.0 plugs. Anyone have any suggestions on where to look for those? From what I've been reading about master cylinder failure symptoms, however, it doesn't sounds like a master cylinder problem. Most notably, the absence of the pedal going to the floor even with prolonged brake application seems to speak to an intact master cylinder.
I'm wondering if there are any other pieces in the hyrdraulic system that can introduce compliance that an mimic air in the system. I'm thinking of rubber hoses or other non-steel lines. The lines that go from the wheel wells to the calipers do not appear to be simple steel lines. Can these introduce compliance in the system? Has anyone ever replaced those pieces?
Although this still sounds like air in the system, I'd be astounded if 3 flushes with 2 different fluids is not sufficient to adequately bleed the system (with a Motiv PowerBleeder). So now I'm considering other possibilities.
Sounds like I need to find the M10 x 1.0 plugs. Anyone have any suggestions on where to look for those? From what I've been reading about master cylinder failure symptoms, however, it doesn't sounds like a master cylinder problem. Most notably, the absence of the pedal going to the floor even with prolonged brake application seems to speak to an intact master cylinder.
I'm wondering if there are any other pieces in the hyrdraulic system that can introduce compliance that an mimic air in the system. I'm thinking of rubber hoses or other non-steel lines. The lines that go from the wheel wells to the calipers do not appear to be simple steel lines. Can these introduce compliance in the system? Has anyone ever replaced those pieces?
I'm wondering if it could be the lines. But I will check the axle nuts this weekend as well. Based on the symptoms (hard pedal even with prolonged pressing on the brake pedal) it seems less likely to be a master cylinder issue. I did find some M10 x 1.0 bolts as suggested to plug the master cylinder to isolate the cylinder from the lines/calipers/pads/rotors but I wonder if I'll be able to really test under similar circumstances since the pedal seems to get soft after driving for a while. I guess I can see if it exhibits the same behavior while just sitting in driveway. But that test is not nearly as entertaining!
It was suggested I check the axle nuts. I found a recommended torque setting of 250 ft. lbs. somewhere. The front axle nuts are 30 mm. The rear axle nuts are 32 mm. Both rear nuts were loose. Both were tightened to 250 ft. lbs. Haven't had a chance to get it on the road yet. Was going to take it to work, but a weak battery prevented me from starting up. Charged the battery today. Will try again tomorrow AM.
High five and a pat on the back to doug_porsche for suggesting a check on the torque of the axle nuts. The rears were indeed loose (the fronts were tight) and tightening them seems to have cured the soggy initial brake pedal feel. Took it for a little shake-down run today. I will have a little bit more of an opportunity tomorrow to test it under a wider variety of circumstances before passing final judgement. But at least for now, the brake pedal is consistently firm even on the first depression of the brake pedal! Whew.
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